The introduction of the tyrosine kinase inhibitor (TKI) imatinib has revolutionised the outlook of chronic myeloid leukemia (CML); however, a significant proportion of patients develop resistance through several mechanisms, of which acquisition of <i>ABL1</i> kinase domain mutations is prevalent. The gene discussed is ABL1; the disease is chronic myelogenous leukemia, BCR-ABL1 positive.