The Sister Study will prospectively examine environmental and familial risk factors for breast cancer and other diseases in a cohort of 50,000 sisters of women who have had breast cancer. Such sisters have about twice the risk of developing breast cancer as other women. The frequency of any relevant genes and shared risk factors will also be higher. Studying sisters will enhance our ability to understand the interplay of genes and environment in breast cancer risk and to identify potentially preventable risk factors. We will assess exposures before the onset of disease, thus avoiding biases common to retrospective studies. The study will create a framework from which to test new hypotheses as they emerge.

Cancer-free sisters aged 35-74 are being recruited nationally. A total of 29,000 enrolled through October 2006; 21,000 more women are needed. The study aims to include a divers group of women. Women of color, older women, and women who work in trades are especially encouraged to enroll. Additional information about the study and how to enroll can be found at www.sisterstudy.org.

Data on potential risk factors and current health status will be collected with telephone interviews and self-completed questionnaires. Blood, urine, and environmental samples will be collected and banked for future use in nested studies of women who develop breast cancer (or other diseases) and a sample of those who don't. The cohort will be followed prospectively for 10 or more years. Annual questionnaires will update medical history and changes in exposures. About 300 new cases of breast cancer are expected to occur in the cohort each year. Analyses will assess the independent and combined effects of environmental exposures and genetic polymorphisms that affect estrogen metabolism, DNA repair, and response to specific environmental exposures. Future analyses will focus on known and potential risk factors (e.g. smoking, occupational exposures, alcohol, diet, obesity) and include measurement of phthalates, phytoestrogens, insulin, growth factors, micro-nutrients, and genes. The cohort will also be used to explore risk for other diseases (e.g. heart disease, osteoporosis, other hormonal cancers, and autoimmune diseases) that are important for women.