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Abstract Background Formation and repair of DNA single-strand breaks are important parameters in the assessment of DNA damage and repair occurring in live cells. The 'Fluorimetric Detection of Alkaline DNA Unwinding (FADU)' method [Birnboim HC, Jevcak JJ. Cancer Res (1981) 41:1889–1892] is a sensitive procedure to quantify DNA strand breaks, yet it is very tedious to perform. Results In order (i) to render the FADU assay more convenient and robust, (ii) to increase throughput, and (iii) to reduce the number of cells needed, we have established a modified assay version that is largely automated and is based on the use of a liquid handling device. The assay is operated in a 96-well format, thus greatly increasing throughput. The number of cells required has been reduced to less than 10,000 per data point. The threshold for detection of X-ray-induced DNA strand breaks is 0.13 Gy. The total assay time required for a typical experiment to assess DNA strand break repair is 4–5 hours. Conclusion We have established a robust and convenient method measuring of formation and repair of DNA single-strand breaks in live cells. While the sensitivity of our method is comparable to current assays, throughput is massively increased while operator time is decreased.

In 1928, Hugo Friedrich Kufs reported on a family with cerebral, retinal, and cutaneous cavernous malformations. Since then, more than 300 families with inherited cavernous malformations have been reported. Genetic studies showed three loci, on chromosomes 7q21-q22 (with the gene CCM1), 7p15-p13 (CCM2), and 3q25.2-q27 (CCM3). The gene product of CCM1 is Krit 1 (Krev interaction trapped 1), a protein interacting with angiogenesis by various mechanisms. Recently, CCM2 has also been identified; its product is a protein which might have a function similar to that of Krit 1. However, the CCM3 gene has still not been found. In this study, we present clinical and genetic findings on 15 German families.

The rare observation of different karyotypes in monozygotic (MZ) twins, i.e., heterokaryotic monozygosity, occurs due to chromosomal aberration in one of the twins after separation of the embryos. We report on the differences of heterokaryotic MZ Turkish twins who are discordant for Ullrich-Turner syndrome. Chromosomal analyses from peripheral lymphocytes revealed a 45,X/46,XX mosaicism in both twins. FISH analyses of buccal smears showed 99% of nuclei 45,X in twin A and 98% of nuclei 46,XX in twin B. These results are consistent with a non-mosaic 45,X and 46,XX karyotype, respectively. The girls showed a different growth pattern in the first years. As their genotype should be identical except for the number of X chromosomes, the difference in phenotype may be a pure result of loss of one X chromosome in the affected girl. Special interest is set on the spontaneous and growth hormone induced growth of the twins., (Copyright 2004 Wiley-Liss, Inc.)

Fluorescence in-situ hybridization with human whole chromosome painting probes (WCPs) was applied to compare the karyotypes of members of five bat families. Twenty-five evolutionarily conserved units (ECUs) were identified by ZOO-FISH analysis. In 10 of these 25 ECUs, thorough GTG-band comparison revealed an identical banding pattern in all families studied. Differences in the remaining ECUs were used as characters to judge the phylogenetic relationships within Chiroptera. Close relationships were found between Rhinolophidae and Hipposideridae. Also closely related are the representatives of the yangochiropteran families Phyllostomidae (genus studied: Glossophaga, Volleth et al. 1999), Molossidae and Vespertilionidae. All microchiropteran species studied here share four common features not found in the megachiropteran species Eonycteris spelaea. Two of these are considered as derived characters with a high probability of parallel evolution. On the other hand, Eonycteris shares one common, probably derived feature with the rhinolophoid families Rhinolophidae and Hipposideridae and an additional one only with Hipposideridae. At the moment, the relationships between Yangochiroptera, Rhinolophoidea and Megachiroptera must be left in an unsolved trichotomy. Comparison of neighboring segment combinations found in Chiroptera with those found in other mammalian taxa revealed six synapomorphic features for Chiroptera. Therefore, for karyological reasons, monophyly of Chiroptera is strongly supported.

Deletions within HSA band 4p16.3 cause Wolf-Hirschhorn syndrome (WHS), which comprises mental retardation and developmental defects. A WHS critical region (WHSCR) of approximately 165 kb has been defined on the basis of 2 atypical interstitial deletions; however, genotype-phenotype correlation remains controversial, due to the large size of deletion usually involving several megabases. We report on the first known patient with a small de novo interstitial deletion restricted to the WHSCR who presented with a partial WHS phenotype consisting only of low body weight for height, speech delay, and minor facial anomalies; shortness of stature, microcephaly, seizures and mental retardation were absent. The deletion was initially demonstrated by FISH analysis, and breakpoints were narrowed with a "mini-FISH" technique using 3-5 kb amplicons. A breakpoint-spanning PCR assay defined the distal breakpoint as disrupting the WHSC1 gene within intron 5, exactly after an AluJb repeat. The proximal breakpoint was not found to be associated with a repeated sequence or a known gene. The deletion encompasses 191.5 kb and includes WHSC2, but not LETM1. Thus, manifestations attributable to this deletion are reduced weight for height, minor facial anomalies, ADHD and some learning and fine motor deficiencies, while seizures may be associated with deletions of LETM1., (Copyright 2001 Wiley-Liss, Inc.)

The presence of a monozygotic twin gestation with discordant sex of the twins is a very rare constellation, which is referred to as heterokaryotypic monozygotic pregnancy. This constellation can develop either due to a chromosomal aberration after twinning or is - as in the following case - due to a mitotic error before twinning and an unequal distribution of mosaicism in both embryos. So far the diagnosis of heterokaryotypic monozygotic pregnancy has always been made postnatally, with only one exception (Gonsoulin et al., 1990). In this case we suspected the presence of monozygotic twins ultrasonically because of the chorionic and amniotic membrane characteristics. Surprisingly the sex of the fetuses was discrepant. As one of them had hydrops and a structural heart defect, we carried out an amniocentesis, which revealed mosaicism [45,X/46,X,i(Y)(p10)] of both fetuses. The female fetus with a predominant 45,X set of chromosomes and the typical intrauterine signs of the Ullrich-Turner syndrome (massive hygroma colli, hydrops fetalis and multiple cardiac defects) died during the 25th week of gestation due to cardiac decompensation. The other fetus appeared to be male with a predominance of a 46,X,i(Y)(p10) set of chromosomes and was born a few days after the intrauterine death of the hydropic fetus. In conclusion, our observation shows that ultrasonic evidence of discordant fetal sex in twins does not necessarily exclude monozygosity., (Copyright 2000 John Wiley & Sons, Ltd.)

Background: In the case of abnormal ultrasound findings, abnormal serum-screening and age-risk in advanced pregnancy a rapid diagnosis or exclusion of a chromosomal aneuploidy of the fetus is of great value for the clinical management. With fluorescence in situ hybridization (FISH) on uncultured amniotic fluid cells the detection of the most common aneuploidies, which account for about 2/3 of all chromosomal aberrations [1], is possible within 24 hours. The aim was to evaluate if the FISH-technique in combination with karyotyping after cell culturing could replace other methods like diagnostics form umbilical cord blood or placental biopsies., Materials and Methods: For the FISH assays commercially available directly with fluorochromes labelled DNA-probes (Vysis, Stuttgart) were used. FISH assays were performed on amniotic fluid samples from pregnancies at risk for fetal chromosome aberrations parallel to standard cytogenetic analysis. The method was performed on 230 samples of amniotic fluid. We tried to optimize the method concerning preparation of the cell material, the denaturation- and hybridization-steps and well as stringency of post-hybridization washes., Results: All trisomies 13, 18, 21 and the sex chromosome aneuploidies (n = 34) which were diagnosed by conventional cytogenetics were identified correctly by FISH analysis with the exception of one case of trisomy 21 mosaicism, in which hybridization failed. As structural chromosome aberrations and mosaicisms cannot be detected with this method, six additional chromosome aberrations were identified exclusively by cytogenetic analysis. The mean frequency of nuclei with abnormal signal pattern in the aneuploid cases was 89%. A minimum of 50 nuclei for each DNA-probe could be counted in 86% of the samples. The results of 12 cases were classified as uninformative, because only less than 15 hybridized nuclei or no hybridization signals could be scored. Maternal contamination was found in 17.4% of the samples., Conclusions: In clinical cases with a high risk for an abnormal fetal karyotype and the need of quick clinical consequences, methods which make possible a karyotyping within shortest time should be preferred to amniocentesis and FISH-analysis, because Chromosomal mosaicism and structural aberrations, which represent up to 20% of all chromosomal abnormalities in this group, cannot be detected, uninformative cases can occur in up to 15% of all investigated samples and There is a risk for false-negative results through contamination of the sample with cells of maternal origin. In comparison with methods which permit rapid karyotyping from umbilical cord blood or placental biopsies, a delay in the diagnostic procedure has to be accepted, when the result of the FISH-analysis has to be confirmed by cell culturing and standard cytogenetic analysis.

Ichthyosis follicularis, atrichia, and photophobia (IFAP) are typical features of a rare neuroichthyosis termed IFAP syndrome. We demonstrate the ultrastructural findings of the eyes from a 33-year-old patient with IFAP syndrome. Clinically, eyebrows and eyelashes were absent from birth, and photophobia was noted at the age of 1 year. The globes measured 28 and 29 mm, respectively, and both eyes showed a posterior staphyloma. Histopathologically, bilateral centrally located subepithelial avascular corneal scarring with secondary corneal amyloid deposition was found. In addition to already described ocular abnormalities in IFAP syndrome we demonstrate ultrastructural anomalies of desmosomes and tonofilaments in corneal epithelium; defects of basement membrane, Bowman layer, and anchoring fibrils; secondary corneal amyloid deposition; and keratocyte degeneration. A defective tear film, recurrent atopic keratoconjunctival inflammations, or a primary anomaly of corneal epithelial adhesion are potential causes for the corneal defects. Photophobia is most likely due to corneal abnormalities.

Unlabelled: We report on a male infant with ambiguous genitalia (scrotal hypospadias, sinus urogenitalis) trisomic for 8q23-ter and monosomic for 9p23-ter, who shared craniofacial and other abnormalities with either phenotype. Gonadal histology was nearly normal for age. Normal endocrinological findings and exclusion of mutations in SRY, androgen receptor and alpha-reductase genes point to supplementary gene(s) located in 9p2305-ter, haplo-insufficiency (by deletion) of which is expected to cause defective male morphogenesis., Conclusion: This observation lends further support to the hypothesis that genetic factors are located at 9p23-ter which are involved in normal sex determination.

Diamond-Blackfan anemia (DBA) is a rare pure red-cell hypoplasia of unknown etiology and pathogenesis. A major DBA locus has previously been localized to chromosome 19q13.2. Samples from additional families have been collected to identify key recombinations, microdeletions, and the possibility of heterogeneity for the disorder. In total, 29 multiplex DBA families and 50 families that comprise sporadic DBA cases have been analyzed with polymorphic 19q13 markers, including a newly identified short-tandem repeat in the critical gene region. The results from DNA analysis of 29 multiplex families revealed that 26 of these were consistent with a DBA gene on 19q localized to within a 4.1-cM interval restricted by loci D19S200 and D19S178; however, in three multiplex families, the DBA candidate region on 19q13 was excluded from the segregation of marker alleles. Our results suggest genetic heterogeneity for DBA, and we show that a gene region on chromosome 19q segregates with the disease in the majority of familial cases. Among the 50 families comprising sporadic DBA cases, we identified two novel and overlapping microdeletions on chromosome 19q13. In combination, the three known microdeletions associated with DBA restrict the critical gene region to approximately 1 Mb. The results indicate that a proportion of sporadic DBA cases are caused by deletions in the 19q13 region.

Interruption of the aortic arch (IAA) is a severe malformation of the heart with known association to DiGeorge syndrome (DGS) and 22q11.2 hemizygosity. The aim of this study was to establish incidence and significance of 22q11.2 hemizygosity in an unbiased sample of patients with IAA. All 15 children with IAA who were referred to our hospital in a 3-year period were tested by chromosome and fluorescence in situ hybridization (FISH) analysis with the probes D22S75, Tuplel, and cHKAD26 and by a set of 10 simple tandem repeat polymorphic (STRP) markers. In nine of 11 children with IAA type B, 22q11.2 hemizygosity was demonstrated by FISH and STRP analysis, but in none of the four children with type A. In all but one child, deletion size was approximately 3 Mb. The girl with the smaller deletion of approximately 1.5 Mb differed because of an Ullrich-Turner syndrome-like phenotype and severe T-cell defect. Additionally, in one patient with phenotypic signs of DGS, a small deletion distal to the known DGS region containing the marker D22S308 was suspected by STRP analysis. One deletion was shown to be inherited from a healthy father and one IAA type A recurred in a sib. T-cell anomalies were evident in eight of the nine children with classical deletion, five of whom suffered also from hypoparathyroidism. With respect to cause and clinical course, IAA type A and B were shown to represent different entities. This study showed that variable symptoms of 22q11.2 hemizygosity may cluster.

The syndrome of ichthyosis follicularis, alopecia, and photophobia (IFAP) is an uncommon neuroichthyosis described in only 10 males so far. We report on a man with congenital ichthyosis and alopecia with apparently normal development in early infancy. Photophobia and generalized myoclonicastatic seizures began during or after the first year of age and were associated with progressive impairment of motor skills and mental abilities. He died at 33 years of age. Neuropathological findings showed an unusual deformation of the temporal lobes and olivocerebellar atrophy. Cytogenetic and molecular studies did not uncover deletions in either Xp22.2 to 3 or in Xq27.3 to qter.

Although in vitro protein synthesis is a rapid method to screen for translational stops in the adenomatous polyposis coli (APC) gene, truncating mutations at the 5' most end are at risk of being overseen due to their small size. The authors describe a reverse transcriptase-polymerase chain reaction (RT-PCR)-based protein truncation test specifically designed for detecting truncated polypeptide chains of less than 10 kDa. Using this detection system, three novel germline mutations in familial adenomatous polyposis (FAP) patients were identified, i.e. a Gly101 Ter non-sense mutation in exon 3, an exon 4 splice acceptor mutation and a 555delC deletion in exon 5. Morever, a patient manifesting congenital hypertrophy of the retinal pigmented epithelium (CHPRE) was detected with an Arg232Ter mutation in exon 6. This is, to the authors' knowledge, the fourth exception to the rule that FAP patients manifesting CHRPE harbour genetic alternations downstream from APC exon 9. Hence, an alternative hotspot for non-sense mutations associated with CHRPE appears to encompass the codons 215, 216 and 232. Patients reported in this study, exhibited relatively mild clinical symptoms with respect to the age of onset of malignancy (> 50 years of age) and the number of polyps (70-100 adenomas). However, manifestation of severe duodenal adenomatosis was independent of the attenuated colorectal FAP phenotype.

Deletions within chromosome band 22q11.2 are associated with a variety of conditions, although a simple genotype-phenotype correlation has not been established so far. Environmental factors, chance events, or a second hit theory were supported by two observations of monozygotic twins with 22q11.2 deletions and discordant phenotypes [Goodship et al., J Med Genet 1995;32:746-748; Fryer, J Med Genet 1996;33:173]. We present monozygotic twins concordant for 22q11.2 deletion and Cayler syndrome, favoring the view that there exists a predominant genetic determination of the del 22q11.2 phenotype. As these twins are diamniotic and dichorionic, they may offer a more reliable insight in genetic phenotype determination than the other published, probably monochorionic, twins who may have a discordant malformation by twinning itself.

Autosomal dominant juvenile open angle glaucoma (JOAG) is an early-onset form of primary open angle glaucoma (POAG), which has been linked to chromosome 1q21-q31. Recently, mutations in the trabecular meshwork inducible glucocorticoid response gene (TIGR), one of the candidate genes mapped in this region, were identified in glaucoma patients of several families. We screened for mutations of the TIGR gene in two German families with JOAG and in 100 unselected sporadic cases of POAG. In the first family we identified a Pro370Leu mutation and in the second family a Gly367Arg mutation cosegregating with the glaucoma phenotype. No pathogenic mutation was found in 100 sporadic cases but a Tyr347Tyr polymorphism was found in two patients. Furthermore, fluorescence in situ hybridization (FISH) analysis was used to map a TIGR-specific yeast artificial chromosome to 1q24.3-q25.2.

We report on a malformed stillborn with deletion 3p subsequent to direct paracentric insertion (intraarm shift) in the normal father which had been first mistaken for paracentric inversion. The corrected diagnosis was supported by FISH of mapped markers on metaphase chromosomes. In addition we looked for recombinants in sperm. This observation reminds similar cases that had been considered exceptions to the expected meiotic recombination of paracentric inversions and points to a cytogenetic pitfall. Published deletions and paracentric inversions in 3p are briefly quoted.

We report the prenatal findings of mos 46,XY/46,XY,i(20q) after amniocentesis. The propositus presented with two epidermal scalp scars, retrobulbar orbital cysts, and dyssegmentation of the thoracic spine. The abnormal cell line was discovered in cells cultured from the proximal umbilical cord and--by fluorescence in situ hybridization (FISH)--in interphase nuclei from buccal epithelium and urinary sediment but not from the placenta, lymphocytes, or skin fibroblasts.

We report on the prenatal diagnosis of bilateral fetal cataract in the 17th week of pregnancy as the first ultrasonographic sign of a Walker-Warburg syndrome in a familial case. No other anomalies could be detected by ultrasonographic examination at that time. Both the bilateral fetal cataract and specific abnormalities of a Walker-Warburg syndrome were confirmed after the termination of the pregnancy in the second trimester.

We report a child with a duplication-deficiency subsequent to t(15;20)(q25.2;p12.2), transmitted in at least 5 generations, who showed features of 15q- syndrome. We speculate that brachydactyly--most likely because of brachymesophalangism--is a feature of the phenotype of this chromosomal aberration and points to candidate gene(s) in this region. A similar brachydactyly was, however, reported with dup(20p1-pter).

It has previously been shown that, in the heterozygous state, mutations in the SOX9 gene cause campomelic dysplasia (CD) and the often associated autosomal XY sex reversal. In 12 CD patients, 10 novel mutations and one recurrent mutation were characterized in one SOX9 allele each, and in one case, no mutation was found. Four missense mutations are all located within the high mobility group (HMG) domain. They either reduce or abolish the DNA-binding ability of the mutant SOX9 proteins. Among the five nonsense and three frameshift mutations identified, two leave the C-terminal transactivation (TA) domain encompassing residues 402-509 of SOX9 partly or almost completely intact. When tested in cell transfection experiments, the recurrent nonsense mutation Y440X, found in two patients who survived for four and more than 9 years, respectively, exhibits some residual transactivation ability. In contrast, a frameshift mutation extending the protein by 70 residues at codon 507, found in a patient who died shortly after birth, showed no transactivation. This is apparently due to instability of the mutant SOX9 protein as demonstrated by Western blotting. Amino acid substitutions and nonsense mutations are found in patients with and without XY sex reversal, indicating that sex reversal in CD is subject to variable penetrance. Finally, none of 18 female patients with XY gonadal dysgenesis (Swyer syndrome) showed an altered SOX9 banding pattern in SSCP assays, providing evidence that SOX9 mutations do not usually result in XY sex reversal without skeletal malformations.

We report on the prenatal discovery of 3 up to 7 accessory small marker chromosomes per cell with postnatal confirmation in various tissues. By FISH it could be shown that every marker had a different origin.

Although the majority of fragile-X patients demonstrate methylation and a much-expanded CGG repeat region in the 5'-untranslated region of exon 1 of the FMR1 gene, exceptional cases have been reported to be due to deletions. However, fine mapping of the deletion breakpoints is still lacking and so far the underlying mechanism is unknown. We identified a fragile-X patient mosaic for a full mutation and a microdeletion. The microdeletion spans 486 bp, involving 168 bp upstream from the CGG repeat region, the entire CGG repeat region, exon 1, and 138 bp of the first intron of the FMR1 gene. In contrast to previous reports, the 5' breakpoint does not fall into the hotspot region. The proximal breakpoint, 5'-GTGGTT/T-3', and the distal breakpoint, 5'-GTTGTT/GG-3', can be characterized as chi-like elements and are flanked by direct tandem repeats. Mosaicism of a full mutation and the microdeletion in the DNA of the patient's leukocytes indicates the mitotic origin of the microdeletion. Since the microdeletion allele is unmethylated, it can be concluded that it is not derived from the methylated full mutation but from an unmethylated premutational allele.

Two new cases of del(2)(q37.1) and one case of partial trp(2)(q37) are studied by FISH with cosmid probes from the COL6A3 and PAX3 genes mapped in 2q37.3 and 2q36, respectively. While the PAX3 gene dosage appeared unaffected, the COL6A3 gene was found to be deleted and triplicated, respectively. This finding could explain features of connective tissue disorders such as joint laxity and hypotonia or joint stiffness and epiphyseal dysplasia, particularly documented by congenital dislocation of the radial head. Karyotype-phenotype correlations with reference to published cases are discussed.

We report on a male infant with an unusual type of acrocephalosyndactyly presenting with unilateral coronal craniosynostosis, cutaneous syndactyly of toes 2 and 3, loss of distal triradii, and transverse alignment of dermal ridges of the palm suggesting syndactyly, atresia of the proximal jejunum, and anal stenosis.

Two unpublished cases with partial tandem duplication of 12p and one previously published case were studied by fluorescence in situ hybridization using 11 cosmid DNA probes from 12p. We propose that the smallest duplications of 12(p13.2pter) and 12(p13.1p13.33) produce the "trisomy 12p syndrome" which is characterized by heavy birth weight, macrocephaly, muscular hypotonia, short neck, flat face, high forehead, prominent cheeks, large philtrum, short nose with anteverted nostrils, and broad everted lower lip. From a review of the published cases we conclude that gross malformations are lacking in "pure" trisomy 12p, and mental retardation is severe in complete and moderate in partial trisomy 12p. Polydactyly and accessory nipples were found only with almost complete trisomy 12p. Abnormalities of hair growth may be related to a gene at 12p. The sub-band 12p11.21 may be critical for acrocallosal syndrome. Macrocephaly may be due to a metabolic disorder.

Campomelic dysplasia (CMPD1) and autosomal XY sex reversal (SRA1) are caused by mutations in the SRY-related gene SOX9 on 17q. Unexpectedly, the 17q breakpoints in four CMPD1 translocation cases previously analyzed by us and others map 50 kb or more from SOX9. Here, we present clinical, cytogenetic, and molecular data from a new CMPD1/SRA1 patient with t(6;17)(q14;q24). Fluorescence in situ hybridization has shown that the 17q breakpoint in this case maps to the same region as the breakpoints in the other translocation cases, at least 130 kb from SOX9. Likewise, the breakpoints in two of the previously described cases also map more than 130 kb and, as shown by pulsed field gel electrophoresis analysis, at most 400 kb or 690 kb from SOX9. By using a SOX9 coding sequence polymorphism, expression of both SOX9 alleles has been demonstrated by the reverse transcriptase polymerase chain reaction in lymphoblastoid cells from one of the translocation cases.

We report on two unrelated mentally retarded girls aged 14 and 24 years with short stature and strikingly similar craniofacial dysmorphisms. Whether they share the same entity or different unknown syndromes remains an open question.

Microsatellite analysis with 13 microsatellites spread over 18p was performed to determine the origin of the marker chromosome in 9 patients with additional metacentric marker chromosomes. Phenotypes and banding patterns suggested that the markers were isochromosomes 18p. Maternal origin was determined in all 8 cases where both parents were available for study. Six cases showed 3 alleles (one paternal, one maternal each in single and double dose) of informative markers located close to the telomere while markers close to the centromere on 18p were reduced to homozygosity (one paternal allele in single dosage and one maternal allele presumably in triple dosage). A similar result was obtained in the patient with no parents available for examination. The other 2 patients were uninformative for maternal hetero- versus homozygosity, but at some loci the maternal band was clearly stronger than the paternal one whereas the opposite was never observed. Trisomy 18 differs from trisomy 21, XXX and XXY of maternal origin through a preponderance of meiosis II versus meiosis I nondisjunction. Thus, the results of our study and the advanced mean maternal age at delivery of patients with additional i(18p) indicate that in most if not all cases the marker chromosome originates from maternal meiosis II nondisjunction immediately followed by isochromosome formation in one of the 2 maternal chromosomes 18. Possible explanations of these results include a maternally imprinted gene on 18q with a lethal effect if the paternal homologue is lost and a mechanism through which nondisjunction in some cases could be connected with isochromosome formation.

We report two cases with partial trisomy of 10q24.1-ter with concomitant deficiency of 7pter and 4qter, which share aplasia of the optic nerve and malformation of the anterior chamber. A review of published observations of partial trisomies of 10q showed that abnormalities of the eye, and particularly of the orbital region, are frequent, but ophthalmological studies are lacking. We conclude that our findings demonstrate a major effect of this chromosomal imbalance.

The cosegregation of a reciprocal translocation t(17;19) (q11.2;13.2) with neurofibromatosis type 1 in three generations suggested that the breakpoint on chromosome 17 involved the NF1 gene. In order to map the breakpoint, we analysed DNAs of patients using parts of the NF1 gene as probes. Southern analysis revealed that the chromosome 17 breakpoint lies within intron 23 of the NF1 gene. One of the patients of the family developed a non-Hodgkin lymphoma. An additional translocation t(14;20) (q32;13.1) in his B lymphocytes points to a gene on chromosome 20 that is juxtaposed to the IGH locus on 14q32, and that may be of relevance for the development of this tumor type.