ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive: The generation of a constitutively active BCR-ABL1 fusion tyrosine kinase due to chromosome translocation is a major cause of chronic myeloid leukemia (CML) and the development of imatinib, the first BCR-ABL1 tyrosine kinase inhibitor, revolutionized the treatment of CML.