An improved understanding of how leukemia cells grow and become resistant to treatment remains critical for developing more effective therapies. We have identified activating mutations of c-kit at codon 816 (Asp(816) ) from a revertant of the cytokine-dependent acute myeloid leukemia (AML) cell line, MO7e (D816H), and de novo childhood AML (D816N). Following transduction of the mutant c-kit cDNAs, MO7e cells acquire a growth advantage and resistance to apoptosis in response to chemotherapeutic drugs and ionizing radiation, in addition to cytokine-independent survival. Although stimulation of mutant c-kit-bearing MO7e cells with stem cell factor (SCF), a ligand for c-Kit, does not have a significant effect on cell proliferation, SCF further inhibits apoptosis induced by cytotoxic agents. These results suggest a potentially important role of Asp(816) mutations of c-kit in both malignant cell proliferation and resistance to therapy.