Resistance to targeted treatment with the BCR-ABL inhibitor, imatinib, in patients with chronic myeloid leukaemia and Philadelphia-positive (Ph+) acute lymphoblastic leukaemia can occur through the selection of point mutations within the ABL kinase domain, which stop imatinib binding without impairing kinase activity. 1 Dasatinib (BMS-354825) is a second-generation tyrosine kinase inhibitor currently in clinical development for imatinib-resistant or imatinib-intolerant chronic myeloid leukaemia and Ph+ acute lymphoblastic leukaemia. 2 In vitro assays3, 4 and crystallographic studies5 have shown that the less stringent conformational requirements for BCR-ABL binding render dasatinib active against many of the ABL kinase domain mutations seen in patients who are resistant to imatinib. However, novel, inhibitorspecific mutants of BCR-ABL are thought to eventually emerge. 6 Here we report two patients in whom resistance to dasatinib was associated with the emergence of two such dasatinib-specific mutants. Both patients were enrolled in a phase II programme of dasatinib for Ph+ leukaemia resistant to, or intolerant of, imatinib. The first case is an 18-year-old patient with Ph+ acute lymphoblastic leukaemia who had stopped imatinib because of disease recurrence. Mutation analysis at the time of relapse had indicated the presence of a Y253H BCR-ABL mutation, which resulted in resistance to imatinib. Dasatinib treatment was started in May, 2005, at a dose of 70 mg twice a day. The patient showed complete haematological and cytogenetic (0% Ph+ marrow metaphases) response after 3 months of treatment, and only wild-type Bcr-Abl was detectable by direct sequencing at 3, 6, and 9 months. At 12 months, mutation analysis showed that BCR-ABL-positive cells had emerged, which harboured an ACT (adenine, cytosine, thymine) to GCT (guanine, cytosine, thymine) nucleotide change, leading to a threonine to alanine amino acid substitution at residue 315 (T315A; figure 1: top panel). At 13 months, the patient had a haematological relapse, and mutation analysis showed that almost 100% of BCR-ABL-positive cells were harbouring T315A, because only the mutated peak was detectable (figure 1: bottom panel). Dasatinib was stopped and rechallenging with imatinib at a dose of 800 mg per day was attempted. After 1 week from the start of imatinib, white blood-cell count had reduced from 45× 109/L to 1· 9× 109/L. Mutation analysis done after 3 weeks and 6 weeks of imatinib showed that the T315A mutation was still present, and that additional G250E and F317L mutations had been selected. For this reason, and because of disease persistence, imatinib was withdrawn after 50 days. The second case is a 74-year-old patient with Ph+ acute lymphoblastic leukaemia who had discontinued imatinib because of grade IV non-haematological toxic effects. Mutation analysis of a blood sample obtained before the onset of treatment with dasatinib had not shown evidence of ABL kinase domain mutations. Dasatinib was started in January, 2006, at a dose of 140 mg per day. The patient showed complete haematological response after 1 month and a complete cytogenetic response after 3 months of dasatinib treatment. At 6 months, the patient relapsed and sequence analysis detected a TTC