There is epidemiological evidence for the existence of such modifiers for BRCA1 and BRCA2 mutations, since there is familial clustering of the cancer site: carriers in families with ovarian cancer index cases are at higher risk for ovarian cancer and at lower risk for breast cancer than carriers from families with breast cancer index cases (Easton et al, 1995; Antoniou et al, 2003; Simchoni et al, 2006). Here, BRCA1 is linked to breast cancer.