ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive: Chronic myeloid leukaemia (CML) is an uncommon leukaemia defined by the presence of a Philadelphia chromosome. This chromosome is because of reciprocal translocation, t(9;22)(q34;q11), of genetic material between chromosome 9 and chromosome 22, and it contains a fusion gene called BCR-ABL1.