BCR and chronic myelogenous leukemia, BCR-ABL1 positive: The majority of CML patients presents p210 protein, which occurs in a region where there is a 5.8 kb break in the BCR gene, known as major-bcr (M-bcr), between exons 12 and 16 (known as b1-b5), with most breaks associated with M-bcr occurring between exon 13 and 14 (known as b2-b3).