The penetrance estimates for individuals with a pathogenic BRCA1 or BRCA2 variant range from 45 to 85% for breast cancer and from 10 to 65% for ovarian cancer (Petrucelli et al., 1993; van der Kolk et al., 2010), some of which can be explained by a polygenic background (Kuchenbaecker et al., 2017; Lee et al., 2019; Gallagher et al., 2020). Here, BRCA1 is linked to breast cancer.