BCR and chronic myelogenous leukemia, BCR-ABL1 positive: The diagnostic hallmark of the myeloproliferative neoplasm (MPN) of chronic myeloid leukemia (CML) is the Philadelphia (Ph) chromosome resulting in the BCR-ABL1 fusion gene, whereas the most commonly acquired genetic abnormality in the Ph-negative MPN of polycythemia vera, essential thrombocythemia and primary myelofibrosis is the JAK2 V617F mutation.