BCR and chronic myelogenous leukemia, BCR-ABL1 positive: Rearrangements involving the ABL1 gene at chromosome 9q and the BCR gene at 22q result in formation of the Philadelphia (Ph) chromosome, t(9; 22)(q34; q11), and are present in all cases of chronic myeloid leukemia (CML) and in some cases of acute lymphoblastic leukemia (Ph+ ALL).