Samples for the iCOGS stage were drawn from 52 studies participating in BCAC, including 41 from populations of predominantly European ancestry, 9 of Asian ancestry and 2 of African-American ancestry. The majority of studies were population-based or hospital-based case-control studies, but some studies selected samples by age or oversampled for cases with a family history of breast cancer (Supplementary Table 2). Studies were required to provide ~2% of samples in duplicate.
