BCR and chronic myelogenous leukemia, BCR-ABL1 positive: At the BCR locus, the breakpoints occur in the major breakpoint cluster region (M-bcr), which spans a 3 kb region that includes exons 13 and 14 of BCR. All the rearrangements involving both breakpoint regions give rise to a 210 kDa protein, the most common chimeric transcript in CML [16].