Correspondence To

the editor: Varying response to escalating the dose of imatinib in patients with CML who “acquire ” a BCR-ABLM244V mutant allele We report results of monitoring BCR-ABL transcript levels in 6 patients with chronic myeloid leukemia (CML) treated with imatinib mesylate (IM) who were found to have the same BCR-ABLM244V mutant allele. In vitro data suggest that this mutation is moderately resistant to IM, with an inhibitory concentration at 50 % (IC50) of 4.1 M,1 and that resistance may be overcome by dose escalation.2,3 In practice, the response to increasing the doses of IM varied in the 6 patients (Figure 1), an observation that can be interpreted in 1 of 3 ways: (1) the degree to which the mutation confers resistance to IM is very variable in different patients; (2) the presence of a M224V mutant may not always be the direct cause of the resistance; or (3