BCR and chronic myelogenous leukemia, BCR-ABL1 positive: Chronic myelogenous leukaemia was the first human cancer to be associated with a consistent chromosomal abnormality, the Philadelphia chromosome, a translocation that juxtaposes the 3′ sequence from the ABL1 proto-oncogene on chromosome 9 with the 5′ sequence from the BCR gene on chromosome 22.