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We examined the subcellular localization of BRCA1 proteins using immunohistochemical staining with C-terminal (GLK-2 antibody) and N-terminal (Ab-2 antibody) monoclonal antibodies in 44 familial ovarian cancers. Among these, 24 cases were associated with 13 independent germ-line mutations of BRCA1, and loss of heterozygosity (LOH) at one or more BRCA1 microsatellite markers was found in all 21 informative tumors tested. With GLK-2 antibody, cytoplasmic staining was observed in 15 of 16 tumors (93.8%) with mutation in exon 11, and BRCA1 staining was absent in 8 of 8 tumors (100%) with mutation in exons other than exon 11. When immunohistochemical staining was performed with Ab-2 antibody, both nuclear and cytoplasmic staining were observed in 14 of 16 tumors (87.5%) with mutation in exon 11. Interestingly, nuclear staining was observed in 3 of 3 tumors (100%) with mutation downstream of exon 11, even though no staining was detected in 5 of 5 tumors (100%) with mutation upstream of exon 11. On the other hand, in familial ovarian cancers without BRCA1 mutations, nuclear staining or both nuclear and cytoplasmic staining was observed in 18 of 20 specimens (90%) and 20 of 20 specimens (100%) with GLK-2 antibody and with Ab-2 antibody, respectively. These results suggest that an immunohistochemical assay in combination with employing the C-terminal and the N-terminal antibodies appears to have potential as a reliable and useful technique for the screening of BRCA1 mutations, at least to predict the status of mutation, upstream or downstream of exon 11. Key words: Familial ovarian cancer -BRCA1 -Subcellular localization -Splice variant -ImmunohistochemistrySince the cloning of the BRCA1 gene on chromosome 17q21 in 1994, 1) many collaborative laboratories have reported a good many different germ-line mutations of BRCA1, in which 85% of the mutations are frameshifts or nonsense mutations predicted to result in protein truncation. [1][2][3][4][5][6][7][8][9][10][11] The BRCA1 mutation database established by the Breast Cancer Information Core contains nearly 700 mutations. Germ-line mutations of BRCA1 are predicted to be responsible for 45% of breast cancer families and 80% of breast-ovarian cancer families.12) The lifetime risk for mutation carriers is reported to be 85% for breast cancer and 60% for ovarian cancer. 4,13) Recently, we have reported that the expected lifetime risk of ovarian cancer is 80% for Japanese women with germ-line mutations of BRCA1. 14)Although new methodologies for genetic testing have been developed, they are still laborious and expensive because the BRCA1 gene is very large and does not have any mutation hot spots. Considering the high frequency of truncating mutations, we have focused our attention on the possibility that a C-terminal antibody may be unable to 16 To whom requests for reprints should be addressed.