Breast cancer is the most common cancer diagnosed among women in Western countries. While rare mutations in genes such as BRCA1 and BRCA2 confer high risks of developing breast cancer, these account for only a small proportion of breast cancer cases in the general population. Multiple common breast cancer susceptibility variants discovered through genome-wide association studies (GWASs)1, 2 confer small risk individually, but their combined effect, when summarized as a polygenic risk score (PRS), can be substantial.3, 4, 5 Such genomic profiles can be used to stratify women according to their risk of developing breast cancer.6 This in turn holds the promise of improved breast cancer prevention and survival, by targeting screening or other preventative strategies at those women most likely to benefit.