ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive: The break cluster region (BCR)–ABL1 oncoprotein is a tyrosine-kinase that constitutively activates downstream signaling thereby inducing cell proliferation, survival, and clonal expansion in chronic myeloid leukemia (CML) and a cohort of acute lymphoblastic leukemia (ALL) patients.