Small cell lung cancer (SCLC) is an aggressive neuroendocrine cancer that accounts for approximately 16% of new lung cancer diagnoses. Though SCLC is initially responsive to cytotoxic chemotherapy, resistance quickly develops, with 5-year survival rates below 7%, a prognosis that has not improved significantly over the past 30 years. ASCL1 is present in a majority of human-derived SCLC cell lines and, where tested, is required for tumor cell growth. This proneural basic helix-loop-helix (bHLH) transcription factor normally regulates neuronal differentiation during embryonic development. Its requirement in SCLC suggests that strategies to inhibit the function of this transcription factor may represent a novel, targeted SCLC therapy. Proneural bHLH transcription factors, including ASCL1, have a defined lifetime during embryonic development. It has been proposed that proneural bHLH transcription factors are regulated by phosphorylation of a serine residue at a conserved position within the bHLH domain. Phosphorylation at this site has been suggested to function as a switch that rapidly shuts off the activity of ASCL1, overcoming an autoregulatory positive feedback loop that sustains the proliferative capacity of ASCL1 expressing cells. Interestingly, ASCL1 does not appear to be phosphorylated at this site in SCLC. I hypothesize that the phosphorylation of ASCL1 at this conserved site can be induced to inactivate this oncogenic driver in the context of SCLC. The current proposal will test this hypothesis by determining the effect of phosphorylation at this conserved site on the transcriptional activity, DNA binding, and heterodimerization capacity of ASCL1. In addition, I will test whether phosphorylation of ASCL1 at this conserved site can be invoked in SCLC to inhibit tumor growth by replacing wild-type ASCL1 with phosphomimetic variants both in vitro and in vivo. Finally, in vitro phosphorylation assays will be performed with several kinases predicted to phosphorylate ASCL1 at this site, to test the ability of candidate kinase agonists to disrupt SCLC growth both in vitro and in vivo. These experiments have the potential to identify a novel, mechanism-based therapy for this treatment resistant cancer.