ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive: Chronic myeloid leukemia (CML) is uniquely characterized among myeloproliferative neoplasms (MPNs) by the presence of a BCR::ABL1 fusion gene, which arises from the translocation of the long arms of chromosomes 9 and 22, specifically t (9;22) (q34;q11).