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Distinction of primary ovarian epithelial tumors from metastatic adenocarcinomas is challenging for tumors exhibiting mucinous, endometrioid, or mixed endometrioid/mucinous differentiation. Metastatic carcinomas with these types of differentiation can be derived from several sites, including the gastrointestinal tract and the uterus. Most endocervical adenocarcinomas exhibit mucinous and/or endometrioid differentiation; they infrequently metastasize to the ovaries but may simulate primary ovarian tumors [both atypical proliferative (borderline) and carcinoma]. Most are high-risk human papillomavirus (HPV)-related and demonstrate diffuse p16 over-expression due to complex molecular mechanisms by which high-risk HPV transforming proteins interact with cell cycle regulatory proteins. The performance of this expression pattern for identifying metastatic endocervical adenocarcinomas in the ovaries among primary ovarian tumors and other metastatic adenocarcinomas having mucinous and/or endometrioid/endometrioidlike differentiation has not been evaluated. Immunohistochemical expression of p16 was assessed in 195 tumors, including 98 primary ovarian tumors (51 mucinous, 47 endometrioid, and 4 mixed mucinous-endometrioid tumors), 93 metastatic adenocarcinomas of known primary sites (colorectum: 34, endocervix: 19, pancreaticobiliary tract: 17, appendix: 7, stomach: 5), 11 metastatic adenocarcinomas of unknown origin (7 established as noncervical), and 4 adenocarcinomas of uncertain (primary ovarian vs. metastatic) origin. The HPV status of the endocervical adenocarcinomas was determined by in situ hybridization and polymerase chain reaction (when in situ hybridization was negative). Expression was assessed based on the percentage of moderately to strongly positive cells, estimated to the nearest 10%. Mean and median expression values for HPV-positive endocervical adenocarcinomas (99%, 100%; range 90% to 100%) were substantially higher than those for primary ovarian mucinous (5%, 0%; range 0% to 70%) and endometrioid (20%, 10%; range 0% to 100%) tumors, HPV-unrelated endocervical adenocarcinomas (0%, 0%; range 0% to 60%), metastatic adenocarcinomas of unknown origin (11%, 0%; range 0% to 30%), and adenocarcinomas of uncertain (primary ovarian vs. metastatic) origin (40%, 35%; range 0% to 90%); only the 15 HPV-positive endocervical adenocarcinomas and 6 other tumors had values of 80% or greater. Diffuse (>75% positive tumor cells) moderate to strong p16 expression is a sensitive (100%) and specific (97%) marker for identifying HPV-related endocervical adenocarcinomas metastatic to the ovary among the primary ovarian tumors and metastatic adenocarcinomas from other sites that are in the differential diagnosis of ovarian tumors having mucinous and/or endometrioid/endometrioidlike differentiation. p16 is useful as part of a panel of immunohistochemical markers for distinguishing primary ovarian tumors from metastases and, when diffusely positive, can suggest the cervix as a potential primary site for metastatic adenocarcinomas of unknown origin.