FLT3 expression and clinical response. (A) Comparison of pretreatment FLT3 expression in clinically lestaurtinib-responsive (hematologic response or greater) and nonresponsive patients. FLT3 expression was quantified at RNA level in BM MNCs by real-time PCR relative to the stably expressed housekeeping gene S14. Surface FLT3 protein expression was measured flow cytometrically using PE-conjugated anti-CD135. Blasts were selected using a CD45/side-scatter gating strategy. The horizontal bars represent the median expression levels in each group. (B) Changes in surface FLT3 protein expression in 14 patients assayed serially, prior to, and during lestaurtinib treatment.