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COX-2, the isoform of cyclooxygenase inducible by cytokines, mitogens, and growth factors, appears to play an important role in inflammation and carcinogenesis. In the colon, COX-2 overexpression results in cell cycle alterations, and NSAIDs have proven effective in cancer chemoprevention. HNPCC (hereditary nonpolyposis colon cancer) is a clinically defined cancer susceptibility syndrome in which women are also at significantly increased risk for the development of endometrial carcinoma. The purpose of this study was to evaluate expression of COX-2 in benign and malignant endometrium in the context of other cell cycle and proliferation markers, including Ki-67, cyclin D1, and the cyclin-dependent kinase inhibitor, p21. Immunostains with COX-2, Ki-67, cyclin D1, and p21 antibodies were performed on formalin-fixed and paraffin-embedded tissue sections from 40 cases: 10 benign (5 atrophic and 5 proliferative) endometria, 6 hyperplasias (complex without atypia), and 24 endometrioid carcinomas (9 well, 4 moderately, and 11 poorly differentiated). Ki-67 was positive in all proliferative and neoplastic endometria. Cyclin D1 and p21 were both overexpressed in endometrial hyperplasia and endometrioid carcinomas. COX-2 was negative in the nonneoplastic endometrium, stained minimally in the well-differentiated endometrioid carcinomas, and stained most strongly in the moderately and poorly differentiated endometrioid carcinomas. Because cyclin D1 may function as an oncogene, its effects may dominate the usual inhibitory effect of a rising p21. Alternatively, it has been shown that p21 can promote cell cycle function by stabilizing cell cycle complexes. The overexpression of COX-2 in poorly differentiated endometrioid carcinoma and lack of expression in hyperplasia and well-differentiated carcinoma suggests that in this form of cancer, COX-2 may play a role in tumor progression rather than tumor initiation.