Ovarian cancer ranks fifth in both cancer incidence and cancer mortality in U.S. women and has the highest mortality rate among gynecologic cancers, with most patients presenting with late stage metastatic disease. Traditionally, most carcinomas have been treated as though they represent a single disease, and treatments have been based on stage and grade. Data is emerging that the response to treatments may differ, depending on the genetic etiology of the cancer. A better understanding of the molecular events that lead to the evolution of cancer and underlie tumor heterogeneity should lead to more specific treatment regimens. One of the best defined molecular pathways involved in both inherited and sporadic cancer pathogenesis involves the mismatch repair (MMR) pathway, which leads to microsatellite instability (MSI). MSI may result from both genetic (i.e.: germline mutations in the MMR genes, including MLH1, MSH2, and MSH6) and epigenetic (i.e.: MLH1 promoter hypermethylation) mechanisms. It is the purpose of the present proposal to quantify the proportion of ovarian tumors due to the mismatch repair genes and to characterize the tumors in this group. Our ability to classify ovarian cancers by their genetic basis offers promise for improvements in cancer prevention and screening of high risk women, in basing diagnosis and prognosis on molecular markers, and in development of individualized treatments. To date, only a few small studies of MMR in ovarian cancer have been performed and none have been population-based. This is in part due to the difficulty in recruiting large numbers of patients with ovarian cancer who are representative of the population. The proposed study is feasible to conduct only because of the extensive resources already available through the use of data and samples from three existing North American population-based studies, representing the majority of population-based cases in North America to date. This study will include 2200 incident epithelial ovarian cancers based at the Moffitt Cancer Center, Duke Comprehensive Cancer Center, and the University of Toronto and will be the largest collection of its type in the world. Paraffin-embedded tumor samples will be analyzed from all subjects with incident ovarian cancers to perform MSI testing and investigate MMR gene protein expression. In those samples with MSI-H status or with loss of expression of MMR gene proteins, epigenetic (MLH1 promoter hypermethylation) and genetic (germline MMR mutations) will be investigated. The clarification of factors that determine the etiology of MSI-H ovarian tumors are relevant to up to one-fifth of patients who have this deadly disease, hence these data may serve to advance clinical practice in several areas, including risk stratification, behavioral modification, and eventually have therapeutic relevance.