SUMMARY SOX2 is a cell fate–determining transcription factor that is expressed in ~70% of cutaneous and head and neck squamous cell carcinomas (SCCs) in patients. SOX2 is highly enriched in stem cell–like tumor-propagating cells (TPCs), which are located within the basal SCC layer where they can self-renew or differentiate into suprabasal SCC cells without proliferative potential. Although we know SOX2 controls the fate choice between TPC self- renewal and squamous differentiation, it is unclear how its activity is regulated in SCCs and whether these regulatory mechanisms could be developed into therapies for cancer patients. Here, we propose to test the hypothesis that phosphorylation of SOX2 inhibits its activity, perturbs the SCC-specific SOX2-PITX1- TP63 self-renewal circuit that drives clonal expansion and SCC growth, and thereby restores the KLF4- dependent squamous differentiation program in SCCs. Our hypothesis is based on our preliminary studies, which showed that SOX2 can be phosphorylated and that this phosphorylation attenuates its activity in SCC cells. We propose to 1) test if SOX2 phosphorylation inhibits TPC self-renewal, clonal expansion, and SCC growth; 2) identify the kinases and phosphatases that regulate SOX2 phosphorylation and function; and 3) define the molecular mechanisms by which SOX2 activity governs TPC self-renewal, SCC growth, and differentiation in mouse and patient-derived SCC models. We expect our proposed research will explain the fate choice between TPC self-renewal and terminal differentiation on a molecular level and therefore provide new concepts for the rational development of pharmacological approaches that enforce the commitment of TPCs to terminally differentiate into SCC cells without proliferative potential.