BCR and chronic myelogenous leukemia, BCR-ABL1 positive: Chronic myelogenous leukemia (CML) is a myeloproliferative disorder characterized by the fusion gene Bcr-Abl, which is generated by a chromosomal translocation of the Abl gene on chromosome 9 to the Bcr gene on chromosome 22 to give a constitutively active protein tyrosine kinase, BCR-ABL1–5.