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PAX8 has emerged as a useful immunohistochemical marker for epithelial neoplasms of gynecologic origin. Expression of PAX8 in uterine malignant mesodermal mixed tumors (MMMT, carcinosarcoma) has not been characterized in detail. The goal of this study is to evaluate PAX8 expression in uterine MMMTs, with particular attention to its distribution in specific tumor components. Thirty-seven cases were studied. PAX8 expression was assessed by immunohistochemistry and scored separately in the epithelial and mesenchymal components of the tumors. The extent of staining was scored based on the estimated percentage of positive tumor cells as 1+: 1% to 25%; 2+: 26% to 50%; 3+: 51% to 75%; 4+: 76% to 100%. The epithelial component expressed PAX8 in all but 1 tumor, with 92% of tumors displaying 3+ and 4+ extent of staining. The mesenchymal component lacked PAX8 expression in 27 cases (73%) with variable expression in the remaining 10 cases. In addition, 12 tumors contained undifferentiated areas that were not readily classifiable as carcinoma or sarcoma based on morphologic features. Of these, 8 (67%) were negative for PAX8, whereas 4 (33%) demonstrated variable extent of expression. Thus, PAX8 is expressed in the carcinomatous components of nearly all uterine MMMTs (97%), with expression in sarcomatous and undifferentiated components being less common and less extensive. The uniform, extensive expression in the carcinomatous components makes PAX8 a useful marker for diagnosis of carcinomatous metastases of uterine MMMT at extrauterine sites. Its infrequent expression in the sarcomatous and undifferentiated components limits its utility in identifying sarcoma-predominant metastases as gynecologic in origin.