BCR and chronic myelogenous leukemia, BCR-ABL1 positive: The balanced translocation of 5′ segment of the breakpoint cluster region (BCR) gene on chromosome 22 to the 3′ segment of the Abelson leukemia virus (ABL) gene on chromosome 9 in chronic myeloid leukemia (CML), leads to the formation of the BCR‐ABL hybrid fusion gene, which has constitutively upregulated tyrosine kinase activity.