BRCA1 and breast cancer: The three repeatedly occurring mutations in BRCA1 were detected in more than 56% of women with identified alterations in BRCA1/2 genes and in more than 71% of women with alterations in BRCA1. The most frequent mutation was c.5266dupC (5382insC) found in 15 (15.6%) of 96 analyzed BC/OC families and in 18 (40.9%) of 44 BRCA1/2 mutation positive patients.