PROJECT SUMMARY / ABSTRACT OC is the deadliest gynecological cancer in the US. It consists of several histotypes, each biologically distinct with different clinical challenges. In the first three and half years of the parent grant, in spite of challenges imposed by COVID, we were able to make substantial progress towards completion of all Aims. We have published or submitted four papers, with three more to be submitted soon. We were able to develop novel technologies, new bioinformatics tools, generate and analyze epigenomic and transcriptomic data as planned. In particular, we discovered that clear cell ovarian cancer (CCOC) and endometrioid ovarian cancer (ENOC), two closely related OC histotypes, resemble normal endometrial cells at different menstrual cycle phases (proliferative/follicular, secretory/luteal). Traditionally, the cell-of-origin and accumulated genetic mutations are viewed as the most important determinants in initiation and development of cancer. However, this dogma does not apply to the case of CCOC and ENOC, in that both ovarian cancer subtypes arise from the same cell-of- origin (ectopic endometrium/endometrial-like cells) and share common genetic mutations (ARID1A, PIK3CA, KRAS), yet demonstrate drastic differences in cellular phenotype and clinical behavior. Through the case of ENOC and CCOC, we were able to show how cell state (as opposed to cell type) is an underappreciated notion in the discussion of cell of origin, and how non-genetic mechanisms (epigenetic and transcriptional) regulate this process. We also observed substantial molecular heterogeneity in the microdissected tumor compartment, particularly high-grade serous ovarian cancer (HGSOC), suggesting the presence of significant intratumoral cellular heterogeneity in primary human tumors, an issue the parent grant cannot sufficiently address. Therefore, we propose to examine single-cell variations of non-epigenetic regulators of cellular states in primary human OC samples, with novel technologies developed during the parent grant.