Given the high rates of supplement use among women in Canada and the United States, as well as the emerging evidence that vitamin D plays a role in BRCA1-mediated cancers [4,9,10], we conducted a case–control study to investigate the association between supplemental vitamin D and breast cancer among women with a BRCA1 or BRCA2 mutation enrolled in an ongoing longitudinal study of hereditary cancer. The gene discussed is BRCA1; the disease is breast cancer.