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Several recurrent disease-initiating genetic abnormalities define biologically, clinically, and prognostically distinct subsets of disease in B-lymphoblastic leukemia (B-ALL) [[1]]. Coexistence of ETV6/RUNX1 and MLL aberrations in B-cell precursor acute lymphoblastic leukemia discloses a small subclass of BCP-ALL. (D) Metaphase nucleus illustrating derivative chromosomes 9 (black dashed arrow) and 11 (full black arrow) with a KMT2A:MLLT3 fusion and an uninvolved chromosome 11 (white arrow) with metaphase FISH. [Extracted from the article]

• Integrated genomic approach to diagnose myeloid neoplasms should include microarray. • MPseq precisely identifies and characterizes complicated chromosomal rearrangements. • PCM1-JAK2 fusions with aberrant tyrosine kinases have potential for targeted therapy. The 2016 World Health Organization entity 'Myeloid/lymphoid neoplasms with eosinophilia and rearrangement of PDGFRA, PDGFRB or FGFR1 , or with PCM1-JAK2 ' encompasses a group of rare neoplasms that result from the formation of a fusion gene that leads to expression of an aberrant tyrosine kinase. This entity also contains variant JAK2 fusion partners, and detection of this defining event can be facilitated by various cytogenetic and molecular methods. Cryptic rearrangements of 9p24/ JAK2 can be particularly challenging to identify. We describe the use of chromosomal microarray analysis (CMA), fluorescence in situ hybridization (FISH) with a probe for JAK2 , and genomic mate pair analysis to describe a complex karyotype with a t(9;22) that produced a functional BCR-JAK2 fusion, leading to the appropriate diagnosis for the patient. This case highlights the importance of using an integrated genomic approach to fully define complex aberrations to assign proper diagnoses. [ABSTRACT FROM AUTHOR]