Breast cancer tends to cluster in families and up to 15% of all breast cancer cases occur in a family setting that indicates the presence of an inherited risk allele. Germline mutations in two major susceptibility genes BRCA1 and BRCA2 confer a high risk of both breast and ovarian cancer. Other rare mutations in high-risk or moderate-risk genes and common known or predicted low-penetrance SNPs contribute, but half of the suspected inherited risk of breast cancer is still unexplained and warrants further search. Several susceptibility genes have functions in homologous recombination (HR) mediated repair. HR dysfunction (HRD) leaves scares in the tumor genome that may serve as markers for “BRCAness” and used in search for new genetic mechanisms. HRD may also be therapeutically targeted by means of drugs causing synthetic lethality. Thus, the BRCA-field is expanding from risk assessment to treatment prediction and mutation screening is extended from germline to somatic events including HRD.
