Breast secretory activity in nonlactating women, postpartum breast involution, and the epidemiology of breast cancer.

We found an association between the secretory activity of the nonlactating breast and genes related to apocrine function. Based on these findings, we developed a biologic model that relates epidemiologic evidence for an association of reproductive experience with risk of breast cancer to our findings of genetic variation of breast secretory activity and the fact that exogenously derived substances are secreted into breast fluid. The turnover rate of secreted substances was a primary determinant of exposure of the breast epithelium to environmental and endogenous carcinogens and promoters.