Most of the patients with CML possess breakpoints in intron 1 or 2 of the ABL gene and in the main breakpoint cluster region (M-bcr) of the BCR gene, either amongst exons 13 and 14 (b2), or else 14 and 15 (b3) [41]. This evidence concerns the gene BCR and chronic myelogenous leukemia, BCR-ABL1 positive.