Gefitinib inhibits cell viability in the majority of primary patient AML cells. Primary patient AML blasts were isolated from bone marrow aspirate or peripheral blood samples by ficoll separation and treated in a dose-response series with gefitinib. Cell viability was evaluated at 6 days with an ATP-based assay and plotted as a percentage of control cells. Samples were evaluated in replicates of 4. Blasts from peripheral blood mononuclear cells (PBMC) from healthy human donors show markedly decreased response to gefitinib compared with AML samples. One illustrative example is included. FAB class and EC50 evaluation are reported in Table 1. Error bars indicate the standard deviation across 4 replicates.