Relation between high-level FLT3 expression, FLT3 activation, and in vitro sensitivity to PKC412. (A) Immunoprecipitation (IP) analysis of FLT3 in primary infant MLL cells carrying wild-type (patient nos. 1-3 and 5) or mutated (patient no. 4) FLT3, cultured for 4 hours in the absence and presence of 500 nM PCK412. Patient no. 6 represents a noninfant ALL patient. To determine the phosphotyrosine (pTyr) content of FLT3, immunoblots were probed with antiphosphotyrosine (4G10) and with anti-FLT3 to assess FLT3 loading. (B) MTT dose-response curves show the mean cytotoxic response to PKC412 for the individual patients. Error bars represent SEM of duplicate wells. (C) Representation of the FLT3 expression levels for the individual patients. WB indicates Western blot.