BCR and chronic myelogenous leukemia, BCR-ABL1 positive: Chronic myeloid leukemia (CML) is characterized by a balanced reciprocal chromosomal translocation involving the ABL1 gene on chromosome 9 and the BCR gene on chromosome 22, building the BCR-ABL1 fusion gene, which encodes for the constitutively activated tyrosine kinase (TK) BCR-ABL1.