ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive: In cancers such as chronic myeloid leukemia (CML) and several acute lymphocytic leukemias, Abelson tyrosine kinase (Abl1) protein becomes constitutively active due to genetic fusion of the Abl1 gene with the breakpoint cluster region (BCR) gene which is transcribed and translated to the BCR-Abl1 fusion protein.