ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive: In about 75% of BCR-ABL1-positive ALL, but <1% of CML patients, the breakpoint in BCR occurs between exons 1 and 2 in the microcluster region (μ-BCR), generating the e1a2 mRNA fusion that is translated into the p190 oncoprotein.