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Sister Study
The sister study is conducted by the National Institute of Environmental Health Sciences and follows 50,000 women across the United States and Puerto Rico in hopes to find the causes of breast cancer. Each of the participants have/had a sister who was diagnosed with breast cancer, because of their shared environment, genes, and experiences, studying sisters provides a greater chance of identifying risk factors that may help us find ways to prevent breast cancer. The participants are between the ages of 35-74 and will submit to many forms of data collection and questionnaires over a minimum 10-year period.

Participants
The participants in the Sister Study are all women between ages 35-74 who have had/has a sister with breast cancer. 28% of participants were under age 50 at enrollment, 15% of participants had an education of high school or less, 5% of participants were low income, 19% of participants had a mother with breast cancer, 20% of participants had an early age of menarche, and 64% of participants were postmenopausal at enrollment.

Environmental
Environmental factors may include things found in nature that we eat, drink, touch or breathe, as well as man-made factors. There are many environmental risk factors that can increase a woman’s risk of breast cancer. The cohort has found that women who lived in areas with higher levels of lead, mercury, and cadmium in air pollution had a greater chance of developing postmenopausal breast cancer .National Institute of Environmental Health Sciences also concluded that burning wood or natural gas indoors, at least once a week, was associated with a modestly higher risk of breast cancer.2 In the early life exposure assessment, Dr. Mandy Goldberg and her team considered factors that the participants were exposed to while in utero or during infancy. According to their results, early thelarche was associated with multiple pre and postnatal exposures, like secondhand smoke, childhood family income, and soy formula, which resulted in a participant being at higher risk to developing breast cancer.

Genes
While many environmental factors play a role in the cause of breast cancer, genes also play a large role. Most inherited cases of breast cancer are associated with mutations in two genes: BRCA1 and BRCA2. The BRCA genes can be passed from generation to generation. Participants are likely to have a genetic mutation linked to breast cancer if, they have blood relatives that were diagnosed before age 50, both breast and ovarian cancer on the same side of the family, had a relative with triple-negative breast cancer, other cancers in the family, relatives had cancer in both breasts, the are of Eastern European heritage, if they are black and have been diagnosed with breast cancer at age 35 or younger, if a male relative in the family has had breast cancer, or there is a BRCA gene in the family.

Thelarche
Thelarche is the onset of breast development. According to Dr. Goldbergs study, earlier age of thelarche has been linked to increased breast cancer risk. Along with early onset of breast development, the age at menarche is also a risk factor for breast cancer.

Data Collection
Upon enrollment of the Sister Study, all participants agreed to certain data collection guidelines. The baseline data collection includes four self-administered forms, a two-part telephone interview divided into 16 sections and 19 modules on specific occupations, and a form completed by a mobile phlebotomist. Participants had to provide extensive family medical history along with an annual family and individual health update, they also provided detailed follow up questionnaires every two and three years.