BCR and chronic myelogenous leukemia, BCR-ABL1 positive: The characteristic cytogenetic feature of Chronic Myeloid Leukemia (CML) is the fusion of the Abelson murine leukemia (ABL1) gene on chromosome 9 with the breakpoint cluster region (BCR) gene on chromosome 22, known as translocation t(9;22)(q34;q11).