ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive: Expression of the Philadelphia chromosome (Ph), resulting from fusion of the non-receptor tyrosine kinase ABL1 on chromosome 9 with BCR on chromosome 21, is the hallmark of chronic myeloid leukemia (CML), but is also found in 20-30% of acute lymphoblastic leukemia (ALL) cases.