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DNA and Histone 3 Lysine 27 methylation typically function as repressive modifications and operate within distinct genomic compartments. In mammals, the majority of the genome is kept in a DNA methylated state, whereas the Polycomb repressive complexes regulate the unmethylated CpG-rich promoters of developmental genes. In contrast to this general framework, the extra-embryonic lineages display non-canonical, globally intermediate DNA methylation levels, including disruption of local Polycomb domains. Here, to better understand this unusual landscape’s molecular properties, we genetically and chemically perturbed major epigenetic pathways in mouse trophoblast stem cells. We find that the extra-embryonic epigenome reflects ongoing and dynamic de novo methyltransferase recruitment, which is continuously antagonized by Polycomb to maintain intermediate, locally disordered methylation. Despite its disorganized molecular appearance, our data point to a highly controlled equilibrium between counteracting repressors within extra-embryonic cells, one that can seemingly persist indefinitely without bistable features typically seen for embryonic forms of epigenetic regulation.

This descriptive study was conducted to identify the factors affecting problematic internet usage and the socialization level among hearing-impaired adolescents. The study has been done with one hundred twenty six (97 males and 29 females) hearing-impaired students, aged between 14 and 21. The data were collected using Turkish Sign Language, by the researcher who received Sign Language Interpretation Training. ‘The Problematic Internet Usage Scale’ and ‘Socialization Level Scale’ were used as assessment instruments. The results showed that level of the problematic internet usage and the socialization of the participants were at a moderate level with mean scores of 83.8±19.8 on the Problematic Internet Usage Scale and 31.9±8.9 on the Socialization Level Scale. As hearing-impaired adolescents are encouraged to use more internet in order to increase their socialization level, risks as; problematic internet use, keeping young people away from society, and social exclusion should be taken into consideration., Bu tanımlayıcı çalışma, işitme engelli adolesanlarda problemli internet kullanımı ve sosyalleşme düzeylerini etkileyen faktörlerin belirlenmesi amacıyla yapıldı. Çalışma, yaşları 14 ile 21 aralığında olan 126 işitme engelli öğrenci (97 erkek ve 29 kadın) ile yapılmıştır. Veriler, İşaret Dili Tercümanlığı Eğitimi almış araştırmacı tarafından Türk İşaret Dili kullanılarak toplandı. Değerlendirme araçları olarak 'Problemli İnternet Kullanımı Ölçeği' ve 'Sosyalleşme Düzeyi Ölçeği' kullanıldı. Sonuçlar, katılımcıların Problemli İnternet Kullanım Ölçeği’nde 83.8±19.8 ve Sosyalleşme Düzeyi Ölçeği’nde 31.9±8.9 puan ortalamaları ile, problemli internet kullanımının ve sosyalleşme düzeylerinin orta düzeyde olduğunu gösterdi. İşitme engelli adolesanlar sosyalleşme düzeylerini artırmak için daha fazla internet kullanımına yönlendirilirken; problemli internet kullanımı, gençlerin toplumdan uzak tutulması ve sosyal dışlanma gibi riskler göz önüne alınmalıdır.

The complete inventory of regulatory factors in human spliceosomes remains unknown, and many flexibly bound components are not revealed in present spliceosome structures. The intrinsically unstructured C9ORF78 protein was detected in C complex spliceosomes but is not contained in present spliceosome structures. We found a tight interaction between C9ORF78 and the key spliceosome remodeling factor, BRR2, in a large-scale yeast two-hybrid screen, validated by targeted in vitro assays. Affinity purification/mass spectrometry and RNA UV-crosslinking analyses identified several additional C9ORF78 interactors in spliceosomes. High-resolution cryogenic electron microscopy structures revealed how C9ORF78 and the spliceosomal B complex protein FBP21 wrap around the C-terminal helicase cassette of BRR2 in a mutually exclusive manner. Knock-down of C9ORF78 led to global alternative splicing changes, including a substantial usage of alternative NAGNAG 3’-splice sites, at least in part dependent on BRR2. Comparison of our structure to C* complex spliceosomes shows that C9ORF78 could contact several detected interactors from its BRR2 “home base”, in particular the RNA helicase PRPF22, a suggested 3’-splice site regulator. Together our data firmly establish C9ORF78 as a novel, late-stage splicing regulatory protein that takes advantage of a multi-factor trafficking site on BRR2, providing one explanation for the suggested, but puzzling, roles of BRR2 during splicing catalysis and alternative splicing.

Complex, multistep biochemical reactions that routinely take place in our cells require high concentrations of enzymes, substrates, and other structural components to proceed efficiently and typically require chemical environments that can inhibit other reactions in their immediate vicinity. Eukaryotic cells solve these problems by restricting such reactions into diffusion-restricted compartments within the cell called organelles that can be separated from their environment by a lipid membrane, or into membrane-less compartments that form through liquid-liquid phase separation (LLPS). One of the most easily noticeable and the earliest discovered organelle is the nucleus, which harbors the genetic material in cells where transcription by RNA polymerases produces most of the messenger RNAs and a plethora of noncoding RNAs, which in turn are required for translation of mRNAs in the cytoplasm. The interior of the nucleus is not a uniform soup of biomolecules and rather consists of a variety of membrane-less bodies, such as the nucleolus, nuclear speckles (NS), paraspeckles, Cajal bodies, histone locus bodies, and more. In this review, we will focus on NS with an emphasis on recent developments including our own findings about the formation of NS by two large IDR-rich proteins SON and SRRM2.

Nuclear speckles (NS) are among the most prominent biomolecular condensates. Despite their prevalence, research on the function of NS is virtually restricted to colocalization analyses, since an organizing core, without which NS cannot form, remains unidentified. The monoclonal antibody SC35, raised against a spliceosomal extract, is frequently used to mark NS. Unexpectedly, we found that this antibody was mischaracterized and the main target of SC35 mAb is SRRM2, a spliceosome-associated protein that sharply localizes to NS. Here we show that, the core of NS is likely formed by SON and SRRM2, since depletion of SON leads only to a partial disassembly of NS, while co-depletion of SON and SRRM2 or depletion of SON in a cell-line where intrinsically disordered regions (IDRs) of SRRM2 are genetically deleted, leads to a near-complete dissolution of NS. This work, therefore, paves the way to study the role of NS under diverse physiological and stress conditions., eLife digest Most cells store their genetic material inside a compartment called the nucleus, which helps to separate DNA from other molecules in the cell. Inside the nucleus, DNA is tightly packed together with proteins that can read the cell’s genetic code and convert into the RNA molecules needed to build proteins. However, the contents of the nucleus are not randomly arranged, and these proteins are often clustered into specialized areas where they perform their designated roles. One of the first nuclear territories to be identified were granular looking structures named Nuclear Speckles (or NS for short), which are thought to help process RNA before it leaves the nucleus. Structures like NS often contain a number of different factors held together by a core group of proteins known as a scaffold. Although NS were discovered over a century ago, little is known about their scaffold proteins, making it difficult to understand the precise role of these speckles. Typically, researchers visualize NS using a substance called SC35 which targets specific sites in these structures. However, it was unclear which parts of the NS this marker binds to. To answer this question, Ilik et al. studied NS in human cells grown in the lab. The analysis revealed that SC35 attaches to certain parts of a large, flexible protein called SRRM2. Ilik et al. discovered that although the structure and sequence of SRMM2 varies between different animal species, a small region of this protein remained unchanged throughout evolution. Studying the evolutionary history of SRRM2 led to the identification of another protein with similar properties called SON. Ilik et al. found that depleting SON and SRRM2 from human cells caused other proteins associated with the NS to diffuse away from their territories and become dispersed within the nucleus. This suggests that SRMM2 and SON make up the scaffold that holds the proteins in NS together. Nuclear speckles have been associated with certain viral infections, and seem to help prevent the onset of diseases such as Huntington’s and spinocerebellar ataxia. These newly discovered core proteins could therefore further our understanding of the role NS play in disease.

The nucleus of higher eukaryotes is a highly compartmentalized and dynamic organelle consisting of several biomolecular condensates that regulate gene expression at multiple levels (1, 2). First reported more than 100 years ago by Ramon y Cajal, nuclear speckles (NS) are among the most prominent of such condensates (3). Despite their prevalence, research on the function of NS is virtually restricted to colocalization analyses, since an organizing core, without which NS cannot form, remains unidentified (4, 5). The monoclonal antibody SC35, which was raised against a spliceosomal extract, is a frequently used reagent to mark NS since its debut in 1990 (6). Unexpectedly, we found that this antibody has been misidentified and the main target of SC35 mAb is SRRM2, a large (∼300 kDa), spliceosomeassociated (7) protein with prominent intrinsically disordered regions (IDRs) that sharply localizes to NS (8). Here we show that, the elusive core of NS is formed by SON and SRRM2, since depletion of SON leads only to a partial disassembly of NS, while combined depletion of SON together with SRRM2, but not other NS associated factors, or depletion of SON in a cell line where IDRs of SRRM2 are genetically deleted, leads to a near-complete dissolution of NS. This work, therefore, paves the way to study the role of NS under diverse physiological and stress conditions.

Determination of the in vivo binding sites of RNA-binding proteins (RBPs) is paramount to understanding their function and how they affect different aspects of gene regulation. With hundreds of RNA-binding proteins identified in human cells, a flexible, high-resolution, high-throughput, highly multiplexible and radioactivity-free method to determine their binding sites has not been described to date. Here we report FLASH (Fast Ligation of RNA after some sort of Affinity Purification for High-throughput Sequencing), which uses a special adapter design and an optimized protocol to determine protein–RNA interactions in living cells. The entire FLASH protocol, starting from cells on plates to a sequencing library, takes 1.5 days. We demonstrate the flexibility, speed and versatility of FLASH by using it to determine RNA targets of both tagged and endogenously expressed proteins under diverse conditions in vivo.

RNA-binding proteins (RBPs) play important and essential roles in eukaryotic gene expression regulating splicing, localization, translation, and stability of mRNAs. We describe ultraviolet crosslinking and affinity purification (uvCLAP), an easy-to-use, robust, reproducible, and high-throughput method to determine in vivo targets of RBPs. uvCLAP is fast and does not rely on radioactive labeling of RNA. We investigate binding of 15 RBPs from fly, mouse, and human cells to test the method’s performance and applicability. Multiplexing of signal and control libraries enables straightforward comparison of samples. Experiments for most proteins achieve high enrichment of signal over background. A point mutation and a natural splice isoform that change the RBP subcellular localization dramatically alter target selection without changing the targeted RNA motif, showing that compartmentalization of RBPs can be used as an elegant means to generate RNA target specificity., RNA-binding proteins have important roles in gene expression and regulation. Here the authors develop uvCLAP to purify proteins and determine their binding sites without the use of radioactivity.

Determination of the in vivo binding sites of RNA-binding proteins (RBPs) is paramount to understanding their function and how they affect different aspects of gene regulation. With hundreds of RNA-binding proteins identified in human cells, a flexible, high-resolution, high-throughput, highly multiplexible and radioactivity-free method to determine their binding site has not been described to date. Here we report FLASH (Fast Ligation of RNA after some sort of Affinity Purification for High-throughput Sequencing), which uses a special adapter design and an optimized protocol to determine protein-RNA interactions in living cells. The entire FLASH protocol, starting from cells-on-plates to a sequencing library, takes 1.5 days. We demonstrate the flexibility, speed and versatility of FLASH by using it to determine RNA targets of both tagged and endogenously expressed proteins under diverse conditions in vivo.

International audience; The etiological spectrum of ultra-rare developmental disorders remains to be fully defined. Chromatin regulatory mechanisms maintain cellular identity and function, where misregulation may lead to developmental defects. Here, we report pathogenic variations in MSL3, which encodes a member of the chromatin-associated male-specific lethal (MSL) complex responsible for bulk histone H4 lysine 16 acetylation (H4K16ac) in flies and mammals. These variants cause an X-linked syndrome affecting both sexes. Clinical features of the syndrome include global developmental delay, progressive gait disturbance, and recognizable facial dysmorphism. MSL3 mutations affect MSL complex assembly and activity, accompanied by a pronounced loss of H4K16ac levels in vivo. Patient-derived cells display global transcriptome alterations of pathways involved in morphogenesis and cell migration. Finally, we use histone deacetylase inhibitors to rebalance acetylation levels, alleviating some of the molecular and cellular phenotypes of patient cells. Taken together, we characterize a syndrome that allowed us to decipher the developmental importance of MSL3 in humans.

RNA-binding proteins (RBPs) play important and essential roles in eukaryotic gene expression regulating splicing, localization, translation and stability of mRNAs. Understanding the exact contribution of RBPs to gene regulation is crucial as many RBPs are frequently mis-regulated in several neurological diseases and certain cancers. While recently developed techniques provide binding sites of RBPs, they are labor-intensive and generally rely on radioactive labeling of RNA. With more than 1,000 RBPs in a human cell, it is imperative to develop easy, robust, reproducible and high-throughput methods to determine in vivo targets of RBPs. To address these issues we developed uvCLAP (UV crosslinking and affinity purification) as a robust, reproducible method to measure RNA-protein interactionsin vivo. To test its performance and applicability we investigated binding of 15 RBPs from fly, mouse and human cells. We show that uvCLAP generates reliable and comparable data to other methods. Unexpectedly, our results show that despite their different subcellular localizations, STAR proteins (KHDRBS1-3, QKI) bind to a similar RNA motifin vivo. Consistently a point mutation (KHDRBS1Y440F) or a natural splice isoform (QKI-6) that changes the respective RBP subcellular localization, dramatically alters target selection without changing the targeted RNA motif. Combined with the knowledge that RBPs can compete and cooperate for binding sites, our data shows that compartmentalization of RBPs can be used as an elegant means to generate RNA target specificity.

In the absence of DHX9, circular RNAs accumulate and transcription and translation are dysregulated—effects that are exacerbated by concomitant depletion of the RNA-editing enzyme ADAR. In the human genome, there are more than a million copies of the Alu transposable element. Movement of Alu elements is a common source of mutations, but as insertions usually occur in non-coding regions, they are often without discernible effect. Alu elements located near one another in an inverted orientation will form secondary structures that may affect various nuclear processes. Asifa Akhtar and colleagues find that the RNA helicase, DHX9, binds transcribed ‘IRAlus’ (inverted repeat Alu elements). In the absence of DHX9, circular RNAs accumulate, and transcription and translation are dysregulated. These effects are further exacerbated by co-depletion of DHX9 and ADAR p150, an interferon-inducible RNA modification enzyme. The authors conclude that these proteins protect against transposon insertion, which can have deleterious effects on gene expression. Transposable elements are viewed as ‘selfish genetic elements’, yet they contribute to gene regulation and genome evolution in diverse ways1. More than half of the human genome consists of transposable elements2. Alu elements belong to the short interspersed nuclear element (SINE) family of repetitive elements, and with over 1 million insertions they make up more than 10% of the human genome2. Despite their abundance and the potential evolutionary advantages they confer, Alu elements can be mutagenic to the host as they can act as splice acceptors, inhibit translation of mRNAs and cause genomic instability3. Alu elements are the main targets of the RNA-editing enzyme ADAR4 and the formation of Alu exons is suppressed by the nuclear ribonucleoprotein HNRNPC5, but the broad effect of massive secondary structures formed by inverted-repeat Alu elements on RNA processing in the nucleus remains unknown. Here we show that DHX9, an abundant6 nuclear RNA helicase7, binds specifically to inverted-repeat Alu elements that are transcribed as parts of genes. Loss of DHX9 leads to an increase in the number of circular-RNA-producing genes and amount of circular RNAs, translational repression of reporters containing inverted-repeat Alu elements, and transcriptional rewiring (the creation of mostly nonsensical novel connections between exons) of susceptible loci. Biochemical purifications of DHX9 identify the interferon-inducible isoform of ADAR (p150), but not the constitutively expressed ADAR isoform (p110), as an RNA-independent interaction partner. Co-depletion of ADAR and DHX9 augments the double-stranded RNA accumulation defects, leading to increased circular RNA production, revealing a functional link between these two enzymes. Our work uncovers an evolutionarily conserved function of DHX9. We propose that it acts as a nuclear RNA resolvase that neutralizes the immediate threat posed by transposon insertions and allows these elements to evolve as tools for the post-transcriptional regulation of gene expression.

SummaryFgf8 encodes a key signaling factor, and its precise regulation is essential for embryo patterning. Here, we identified the regulatory modules that control Fgf8 expression during mammalian embryogenesis. These enhancers are interspersed with unrelated genes along a large region of 220 kb; yet they act on Fgf8 only. Intriguingly, this region also contains additional genuine enhancer activities that are not transformed into gene expression. Using genomic engineering strategies, we showed that these multiple and distinct regulatory modules act as a coherent unit and influence genes depending on their position rather than on their promoter sequence. These findings highlight how the structure of a locus regulates the autonomous intrinsic activities of the regulatory elements it contains and contributes to their tissue and target specificities. We discuss the implications of such regulatory systems regarding the evolution of gene expression and the impact of human genomic structural variations.

More than forty per cent of the mammalian genome is derived from retroelements, of which about one-quarter are endogenous retroviruses (ERVs). Some are still active, notably in mice the highly polymorphic early transposon (ETn)/MusD and intracisternal A-type particles (IAP). ERVs are transcriptionally silenced during early embryogenesis by histone and DNA methylation (and reviewed in ref. 7), although the initiators of this process, which is essential to protect genome integrity, remain largely unknown. KAP1 (KRAB-associated protein 1, also known as tripartite motif-containing protein 28, TRIM28) represses genes by recruiting the histone methyltransferase SETDB1, heterochromatin protein 1 (HP1) and the NuRD histone deacetylase complex, but few of its physiological targets are known. Two lines of evidence suggest that KAP1-mediated repression could contribute to the control of ERVs: first, KAP1 can trigger permanent gene silencing during early embryogenesis, and second, a KAP1 complex silences the retrovirus murine leukaemia virus in embryonic cells. Consistent with this hypothesis, here we show that KAP1 deletion leads to a marked upregulation of a range of ERVs, in particular IAP elements, in mouse embryonic stem (ES) cells and in early embryos. We further demonstrate that KAP1 acts synergistically with DNA methylation to silence IAP elements, and that it is enriched at the 5' untranslated region (5'UTR) of IAP genomes, where KAP1 deletion leads to the loss of histone 3 lysine 9 trimethylation (H3K9me3), a hallmark of KAP1-mediated repression. Correspondingly, IAP 5'UTR sequences can impose in cis KAP1-dependent repression on a heterologous promoter in ES cells. Our results establish that KAP1 controls endogenous retroelements during early embryonic development.

Abstract 1018 Introduction: There is conflicting data regarding the role of serum ferritin (SF) as an independent prognostic factor for outcome after allogeneic stem cell transplantation (allo-SCT). SF is a surrogate parameter for iron overload and - as an acute phase-protein - can be confounded by many factors. An independent, non-invasive measurement of iron overload is needed. Determining iron content through SQUID correlates well with results of liver biopsies (Nielsen et al. 1998). This method measures the interference of an exteriorly applied small but highly constant magnetic field by the paramagnetic liver storage iron of the patient. Methods: We conducted a single-center retrospective analysis from October 1994 to December 2012, comparing the effect of SF and liver iron content measured by SQUID shortly before transplantation on overall survival (OS), event-free survival (EFS) and transplant-related mortality (TRM) in 143 patients (median age 40 years) undergoing allo-SCT (73% reduced intensity regimen). The diagnoses were subdivided into 4 groups: myelodysplastic syndrome, secondary acute myeloid leukemia (AML), primary myelofibrosis, primary AML and other. Statistical calculations employed Pearson's correlation, ordinal logistic regression, Cox regression analysis and multiple Cox regression analyses with backward elimination. Results: Correlation between SF and SQUID showed a highly significant result of r=0.5 (p < 0.001). The chance of infection was increased 2.4-fold (CI 1.22,4.71) when SQUID values ranged > 1000 μg Fe/g liver. We found similar results for SF > 1000 ng/ml (p 0.003), where the risk of infection was increased 2.87–fold (CI 1.43;5.78). A significant association between SQUID und fungal infection was also seen (p 0.004). For patients with SQUID > 1000 the chance of proven fungal infection versus fungal infection of all other categories was increased 3.08-fold (CI 1.43,6.63). Similarly an association between SF > 1000 and fungal infection could be demonstrated (p 0.001), with a 4.04-fold increased chance of proven fungal infection versus lower fungus categories for patients with SF values > 1000. There was no association between continuous SQUID-, respectively SF values and VOD (veno-occlusive disease) or acute GvHD. The following variables were significantly related with OS: chronic GvHD (HR 0.326, CI 0.13–0.83, p 0.019,), sepsis (p 0.002, HR 2.94 CI 1.48–5.83), new onset cardiac abnormalities (p 0.001, HR 3.21, CI 1.64–6.27), SF > 1000 (p 0.033, HR 2.09, CI 1.06–4.11). For EFS we found statistically significant results for chronic GvHD (p 0.040, HR 0.45, CI 0.21–0.97), sepsis (p 0.012, HR 2.35, CI 1.21–4.58), cardiac abnormalities (p 0.001, HR 2.81, CI 1.50–5.29), SF > 1000 (p 0.016, HR 2.15, CI 1.15–4.10) and time to engraftment (p 0.038, HR 0.934, CI 0.89–1.00). For TRM, statistically significant results were found for sepsis (p 0.005, HR 3.23, CI 1.43–7.29), cardiac abnormalities (p < 0.001, HR 5.21, CI 2.41–11.27), and age (p 0.034, HR 1.04, CI 1.00–1.08). Fungal infection of all categories was not statistically significant (p=1,101), but proven fungal infection in comparison with no fungal infection was found significant (p 0.018, HR 3.12, CI 1.22–7.99. In the multivariate analysis SF and SQUID (categorical and continuous) were not significant factors for OS, EFS or TRM. Conclusion: Our data confirmed previous publications that SF > 1000 increases the risk of infection, moreover fungal infection. In the univariate analysis it is significantly associated with OS and EFS during allo-SCT. As SQUID values correlate well with SF, we could show that SF is indeed a good surrogate parameter for iron overload when measured shortly before allo-SCT. We are now in need of prospective trials investigating the effect of iron chelation before or during allo-SCT on transplant outcome. Disclosures: No relevant conflicts of interest to declare.

Summary Gene expression often requires interaction between promoters and distant enhancers, which occur within the context of highly organized topologically associating domains (TADs). Using a series of engineered chromosomal rearrangements at the Shh locus, we carried out an extensive fine-scale characterization of the factors that govern the long-range regulatory interactions controlling Shh expression. We show that Shh enhancers act pervasively, yet not uniformly, throughout the TAD. Importantly, changing intra-TAD distances had no impact on Shh expression. In contrast, inversions disrupting the TAD altered global folding of the region and prevented regulatory contacts in a distance-dependent manner. Our data indicate that the Shh TAD promotes distance-independent contacts between distant regions that would otherwise interact only sporadically, enabling functional communication between them. In large genomes where genomic distances per se can limit regulatory interactions, this function of TADs could be as essential for gene expression as the formation of insulated neighborhoods., Graphical Abstract, Highlights • Potential for responsiveness to Shh enhancers correlates with contact frequency • Changing distances within Shh TAD has little effect on Shh expression • Inversions bridging the Shh TAD boundaries disrupt folding and regulatory contacts • Decreasing genomic distance can rescue disrupted long-range regulatory interactions, Using systematic genomic rearrangements at the Shh locus, Symmons et al. demonstrate that the topologically associating domain overlapping the locus acts as a functional regulatory mold by promoting contacts between elements that would otherwise be too rare to yield a functional outcome due to the dampening effects of large genomic distances.

Endogenous retroviruses (ERVs) undergo de novo DNA methylation during the first few days of mammalian embryogenesis, although the factors that control the targeting of this process are largely unknown. We asked whether KAP1 (KRAB-associated protein 1) is involved in this mechanism because of its previously defined role in maintaining the silencing of ERVs through the histone methyltransferase ESET and histone H3 lysine 9 trimethylation. Here, we demonstrate that introduced ERV sequences are sufficient to direct rapid de novo methylation of a flanked promoter in embryonic stem (ES) cells. This mechanism requires the presence of an ERV sequence-recognizing KRAB zinc-finger protein (ZFP) and both KAP1 and ESET. Furthermore, this process can also take place on a strong cellular promoter and leads to methylation signatures that are subsequently maintained in vivo throughout embryogenesis. Finally, we show that methylation of ERVs residing in the genome is affected by knockout of KAP1 in early embryos. KRAB-ZFPs, KAP1 and ESET are thus likely to be responsible for the early embryonic instatement of stable epigenetic marks at ERV-containing loci.

Histone acetyl transferases (HATs) play distinct roles in many cellular processes and are frequently misregulated in cancers. Here, we study the regulatory potential of MYST1-(MOF)-containing MSL and NSL complexes in mouse embryonic stem cells (ESCs) and neuronal progenitors. We find that both complexes influence transcription by targeting promoters and TSS-distal enhancers. In contrast to flies, the MSL complex is not exclusively enriched on the X chromosome, yet it is crucial for mammalian X chromosome regulation as it specifically regulates Tsix, the major repressor of Xist lncRNA. MSL depletion leads to decreased Tsix expression, reduced REX1 recruitment, and consequently, enhanced accumulation of Xist and variable numbers of inactivated X chromosomes during early differentiation. The NSL complex provides additional, Tsix-independent repression of Xist by maintaining pluripotency. MSL and NSL complexes therefore act synergistically by using distinct pathways to ensure a fail-safe mechanism for the repression of X inactivation in ESCs. DOI: http://dx.doi.org/10.7554/eLife.02024.001, eLife digest Gene expression is controlled by a complicated network of mechanisms involving a wide range of enzymes and protein complexes. Many of these mechanisms are identical in males and females, but some are not. Female mammals, for example, carry two X chromosomes, whereas males have one X and one Y chromosome. Since the two X chromosomes in females contain essentially the same set of genes, one of them undergoes silencing to prevent the overproduction of certain proteins. This process, which is called X-inactivation, occurs during different stages of development and it must be tightly controlled. An enzyme called MOF was originally found in flies in two distinct complexes—the male-specific lethal (MSL) complex, which forms only in males, and the non-specific lethal (NSL) complex, which is ubiquitous in both males and females. These complexes are evolutionary conserved and are also found in mammals. While mammalian MOF is reasonably well understood, the MSL and NSL complexes are not, so Chelmicki, Dündar et al. have used various sequencing techniques, in combination with biochemical experiments, to investigate their roles in embryonic stem cells and neuronal progenitor cells in mice. These experiments show that MSL and NSL complexes engage in the regulation of thousands of genes. Although the two complexes often show different gene preferences, they often regulate the same cellular processes. The MSL/NSL-dependent regulation of X chromosome inactivation is a prime example of this phenomenon. The MSL complex reduces the production of an RNA molecule called Xist, which is responsible for the inactivation of one of the two X chromosomes in females. The NSL complex, meanwhile, ensures the production of multiple proteins that are crucial for the development of embryonic stem cells, and are also involved in the repression of X inactivation. This analysis sheds light on how different complexes can cooperate and complement each other in order to reach the same goal in the cell. The knowledge gained from this study will pave the way towards better understanding of complex processes such as embryonic development, organogenesis and the pathogenesis of disorders like cancer. DOI: http://dx.doi.org/10.7554/eLife.02024.002