Whereas homozygous inactivation of HR genes is usually embryonic lethal, heterozygous inactivation of for instance BRCA1 and BRCA2 does not interfere with cellular viability and rather predisposes to cancer, including breast and ovarian cancer (Futreal et al., 1994; Miki et al., 1994; Wooster et al., 1994; Lancaster et al., 1996). Here, BRCA1 is linked to cancer.