BCR and chronic myelogenous leukemia, BCR-ABL1 positive: The BCR-ABL oncogene, which results from a reciprocal t(9;22) chromosomal translocation, encodes a chimeric BCR-ABL protein having constitutively activated Abl tyrosine kinase activity, and is the underlying cause of CML (Bartram et al, 1983; Groffen et al, 1984; Lugo et al, 1990).