Chronic myeloid leukaemia (CML) is characterized by the expression of the BCR/ABL1 fusion gene, a constitutively activated tyrosine kinase that commonly results from the formation of the Philadelphia (Ph) chromosome after a t(9;22)(q34;q11) or variant rearrangement. The gene discussed is ABL1; the disease is chronic myelogenous leukemia, BCR-ABL1 positive.