The differences in the Ph chromosome gene product in B-ALL versus CML were found to be the result of a different localization of BCR breakpoints: in B-ALL, they were mapped within the minor breakpoint cluster region (m-BCR) whereas in CML, they fell within the major breakpoint cluster region (M-BCR) (Fig. 2a). Here, BCR is linked to chronic myelogenous leukemia, BCR-ABL1 positive.