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Objectives: The objective of this study was to evaluate SOX17, a transcription factor from the Sry high-mobility group-related box superfamily, as a diagnostic marker to determine site of origin using both whole-tissue sections and tissue microarrays (TMAs)., Methods: SOX17 immunohistochemistry was performed on gynecologic and nongynecologic tissues (N = 1004) using whole-tissue sections and both internally constructed and commercially available TMAs. SOX17 nuclear reactivity was scored as positive or negative on the whole-tissue sections and using the semiquantitative H score method on TMAs., Results: Using both whole-tissue sections and TMAs, SOX17 was positive in 94% (n = 155) of endometrial tumors and 96% (n = 242) of ovarian tumors. All breast cases (n = 241) and vulvar/cervical squamous cell carcinomas (n = 150) were negative. Among 1004 tumors from 20 sites, the only organs with positive tumors were ovary, uterus, and testis., Conclusions: SOX17 is a sensitive and specific marker for gynecologic origin in the tissues tested and may be a valuable adjunct to PAX8 and other commonly used markers to confirm endometrial or ovarian origin. SOX17 expression is lower in mucinous tumors, endocervical adenocarcinoma, high-grade neuroendocrine tumors, and undifferentiated/dedifferentiated endometrial carcinoma., (© The Author(s) 2024. Published by Oxford University Press on behalf of American Society for Clinical Pathology. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.)