ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive: BCR later became the denomination of the gene on chromosome 22 fusing with ABL1. The majority of CML cases are due to a BCR-ABL1 breakpoint, which fuses exons 13 or 14 of BCR with exon 2 of ABL1 (called b2a2 or b3a2, respectively), leading to a BCR-ABL1 oncoprotein of 210 kDa (p210) [10].