The variant involving breakpoints between exons 13 and 14 or exons 14 and 15 of BCR (major BCR) is frequently observed in chronic myelogenous leukemia (CML) and encodes a 210 kDa fusion protein [110], while the variant involving a breakpoint between exons 1 and 2 of BCR (minor BCR) encodes a 190 kDa fusion protein and is frequently observed (90%) in BCR-ABL1-positive pediatric B-ALL. Here, ABL1 is linked to precursor B-cell acute lymphoblastic leukemia.