The Sister Study is a prospective cohort study that enrolled 50,884 US and Puerto Rican volunteers from 2003 to 2009, with follow-up for breast cancer and other health conditions. Eligible women were ages 35 to 74, had never been diagnosed with breast cancer, and had a full or half-sister diagnosed with breast cancer. The Sister Study was approved by the Institutional Review Board (IRB) of the National Institute of Environmental Health Sciences (NIEHS), National Institutes of Health and the Copernicus Group IRB. Further study details are provided elsewhere.32
