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Papillary serous peritoneal carcinoma (PSPC) is histologically indistinguishable from papillary serous ovarian carcinoma (PSOC) with a similar clinical presentation, yet may differ in its carcinogenesis. The purpose of this study was to determine the incidence of allelic loss and the frequency of p53 mutation by p53 overexpression in PSPC compared to PSOC.An allelotype analysis of 26 patients with PSPC was performed using 39 microsatellite markers from 25 chromosomal arms. Thirty-seven previously studied patients with PSOC served as the comparison. P53 mutations were detected by immunohistochemical protein overexpression.There was significantly less LOH in PSPC than PSOC. Both the number of chromosomes with LOH and the proportion of tumors with allelic loss were less frequent. Significant LOH, defined as/=30% of informative tumors having loss at a chromosome locus, was seen on 4 chromosome arms in PSPC: 12p, 17p, 17q, and 18q, compared to 18 arms in PSOC: 4q, 5q, 6p, 6q, 9p, 9q, 12p, 12q, 13q, 15q, 16q, 17p, 17q, 18q, 19p, 19q, 22q, and Xq (P0.001). The median LOH frequency was higher in PSOC than PSPC, 43% versus 33%, respectively (P = 0.013), and more PSOC tumors had LOH than PSPC tumors, 91% versus 65% (P = 0.042). P53 overexpression was detected in 80% of PSPC tumors.LOH occurs less frequently in PSPC compared to PSOC. Chromosomal regions with high frequencies of LOH common to PSPC and PSOC, such as 12p, 17p, 17q, and 18q, may harbor tumor suppressor genes important in the carcinogenesis of both malignancies and likely include p53.