Gene-environment effects
The smaller contributions of many genes throughout the genome are thought to explain population cancer risk, together with environmental factors such as hormone exposure, smoking and aging \cite{Cronin_2018}. At the population level, about 18% of the familial relative risk of breast cancer can currently be explained by mostly common variants identified by genome wide association studies \cite{Michailidou_2017}. Using these variants for polygenic prediction, the 1% of women with the highest predicted genetic risk would have a 3.5-fold (42% lifetime risk) compared to the population average (12% lifetime risk). Such tests are undergoing clinical investigation for their utility in prioritizing early detection and prevention strategies, particularly for those with sparse or unrecorded family history of cancer.