Mostly, the breakpoint on chromosome 22, falling in the so-called major breakpoint cluster region between exons 13 and 14 of the BCR gene (M-bcr), leads to a hybrid BCR–ABL mRNA with a b2a2 or b3a2 junction, which encodes a p210 fusion protein associated with the underlying mechanism, in the chronic phase of CML (3). The gene discussed is BCR; the disease is chronic myelogenous leukemia, BCR-ABL1 positive.