JAK2 and precursor B-cell acute lymphoblastic leukemia: Initial cytogenetic analysis was limited by low mitotic index and poor chromosomal morphology, which precluded identification of the t(8;9)(p22;p24) translocation involving JAK2. Consequently, the JAK2 rearrangement was initially interpreted within the diagnostic and prognostic framework of ALL, i.e. the diagnosis was B-ALL with a BCR::ABL1-like gene expression profile based on the JAK2 rearrangement.