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To investigate at the population level the impact of BRCA1/BRCA2 gene alterations in male breast cancer, we analyzed a population-based series of 25 male breast cancer cases from Florence, Central Italy. We combined mutational screening with the study of germ-line allele transcript levels and of tumor-associated losses of heterozygosity. Screening by protein truncation test and single-strand conformational polymorphism assay, followed by sequencing, revealed 4 pathogenetic mutations (4 of 25 = 16%; 95% confidence interval, 5-37%), 1 in BRCA1 and 3 in BRCA2, including mutations recurring in Central Italy (BRCA1 3345delAG and BRCA2 6696delTC). The a priori probability of carrying a mutation, estimated using BRCAPRO software, showed a good agreement between expected and observed mutations (14% versus 16%). A 7-fold association between germ-line mutations and family history of breast-ovarian cancer emerged. To investigate associations between BRCA1/BRCA2 status and clinicopathological characteristics, we analyzed the histopathological and immunophenotypic parameters of the tumors. A significant association emerged between mutation carrier status and high histological grade (P = 0.02). Furthermore, one BRCA2 carrier was affected with Paget's disease, an extremely rare male breast cancer histotype. Overall, BRCA1/2 mutations were observed to be strongly associated with positive c-erbB-2 immunostaining (P = 0.004). To evaluate germ-line allele expression, we used primer extension assays targeting frequent BRCA1 and BRCA2 polymorphisms. A BRCA2 allele transcript imbalance was found in one of four heterozygotes tested, all of them negative for germ-line mutations. BRCA1 transcript imbalances were not detected in nine heterozygotes analyzed. Losses of heterozygosity at one or more of nine loci in the BRCA2 region were found in 8 of 22 tumors tested. Interestingly, a case that was negative for BRCA1/BRCA2 germ-line mutations and that had a priori mutation probability <10% showed loss of heterozygosity at all three of the intragenic BRCA2 markers analyzed, which could be related to a somatic involvement of BRCA2. No losses of heterozygosity were detected at BRCA1. In conclusion, constitutional BRCA1/BRCA2 mutations accounted for 16% of the male breast cancer cases in this area of Central Italy. The detection of a BRCA2 germ-line transcript imbalance and of a somatic loss of BRCA2 among the cases that resulted negative for germ-line mutations suggests a role of this gene more relevant than indicated by conventional mutational analysis. A distinct pattern of characteristics indicative of aggressive behavior, including high-grade and c-erbB-2 expression, was evident in tumors from germ-line BRCA2 mutation carriers. This suggests that phenotypic characteristics may contribute to the identification of hereditary BRCA2-related male breast cancers and that these tumors might share a unique molecular pathway of cancer progression.