BRCA1 and breast cancer: The frequencies of BRCA1 or BRCA2 mutations among breast-ovarian cancer families varies widely between populations world-wide: Icelandic breast cancer families present almost exclusively BRCA2 mutations [30], roughly equal numbers of BRCA1 and BRCA2 mutations have been described in French Canadian or British breast cancer families [35,36], while a clear prevalence of BRCA1 mutations has been observed in the United States [37,38].