Despite the striking efficacy of tyrosine kinase inhibitors (TKIs) for the treatment of chronic myeloid leukemia (CML), a proportion of patients do not achieve an optimal response and require treatment optimization. To this purpose, three generations of molecules are nowadays available: imatinib (first generation) ; dasatinib, nilotinib and bosutinib (second generation) ; ponatinib (third generation). A variety of mechanisms may underlie lack or loss of response to TKIs, but acquisition of point mutations in the BCR-ABL1 kinase domain (KD) is, at present, the only actionable one4. Each first- and second-generation TKI is known to have a well-defined spectrum of sensitive and resistant mutants. Failure to turn off BCR-ABL1 activity and achieve a rapid and deep clearance of mutant cells not only would result in unsatisfactory clinical response, but would also fuel the acquisition of additional mutations. This, in some patients, would result in a clonal complexity (the so called ‘compound mutants’) that has been shown to be much more difficult to address therapeutically. Screening for mutations is thus recommended both by the European LeukemiaNet (ELN) and by the National Comprehensive Cancer Network (NCCN) in case of failure and warning, that is, whenever a change of therapy is necessary or is to be considered. Capillary (Sanger) sequencing is the current gold standard for diagnostic BCR-ABL1 KD mutation screening. In recent years, however, next-generation sequencing (NGS) has entered routine diagnostic workflows in hematology and oncology as it has proven a powerful and robust technology. In my presentation, I will review the retrospective studies on the use of NGS for BCR-ABL1 KD mutation screening and I will present the results of the first, prospective study (‘NEXT-in-CML’).