CEP-701 potently inhibits constitutively activated FLT3 but does not always induce a cytotoxic response. Immunoprecipitation/Western blotting and CD135-FACS analysis are shown with corresponding MTT cytotoxicity dose-response curves for selected CEP-701–responsive and nonresponsive samples from each group. (A) FLT3/ITD samples (2 responders and 1 nonresponder). (B) FLT3/PM samples (1 responder and 1 nonresponder). (C) FLT3/WT samples (1 responder and 1 nonresponder). Patient numbers (from Tables 1, 2 and 3) are noted next to each curve. Error bars represent SEM of triplicate wells.