In primary infant and childhood ALL blasts, cytotoxic response to FLT3 inhibition is strongly correlated with high-level expression of constitutively activated FLT3. (A) MTT dose-response curves showing mean cytotoxic response to CEP-701 for primary infant and childhood ALL samples, normalized to untreated controls. Error bars represent standard error of the mean (SEM) of triplicate wells. Patient numbers (from Table 2) are noted next to each curve. H indicates Johns Hopkins patient; and J, St Jude patient. (B) FLT3 immunoprecipitation/Western blotting showing FLT3 phosphorylation status (P-FLT3) and total protein expression (FLT3) for each sample from panel A treated for 1 hour with 50 nM CEP-701 (+) or diluent control (-), grouped by CEP-701 responsiveness.