[1] Over 10 years, the study followed 50,000 sisters of women who have had breast cancer in hopes of finding the environmental and genetic causes of the disease.

[7] Using sisters as subjects also provides a higher chance of identifying the potential risk factors because of their shared environment, genetics, and experiences.

[9] Many advocacy groups called for more studies on breast cancer to improve women’s health, as existing research primarily focused on hormones, lifestyle, and reproduction.

[11] The study began by considering the endocrine-disrupting chemical exposures that were of concern to breast cancer advocates, but the researchers wanted to address a range of individual and community-level stressors, such as air pollution and lifestyle factors.

[10] The researchers also aimed to include broader public health concerns such as psychological stress, sleep patterns, diet, and socioeconomic and neighborhood factors.

[10] The first participant was enrolled 20 years ago, in the fall of 2003, and by the spring of 2009, the study surpassed its goal of 50,000 enrollees with a final count of 50,884 women.

[12] By continuing to study these sisters, the researchers hope to have an increased ability to detect disease risks related to both the environment and genes.

[2] Even though they did not complete all parts, this group is followed through record linkage—vital statistics and cancer registries—to assess the outcomes for participants who did and did not fully enroll.

During the study, the researchers also found a correlation between exposure-related changes to DNA and advanced biological aging as a factor for developing breast cancer.

[6] A majority of papers have been published in the Environmental Health Perspectives journal as their research interest overlaps with the Sister Study’s.