The increasing incidence of breast cancer is suspected to be linked to the increased exposure to environmental synthetic estrogens, such as bisphenol A (BPA).We hypothesize that mammary gland exposed to BPA at a susceptible window may lead to its susceptibility to tumorigenesis through a mammary stem cells (MaSC) and/or stem cell niche mediated mechanism.To test this hypothesis, we exposed 21-day-old pubertal Balb/C mice to BPA by gavage at 25 μg/kg/day during puberty for 3 weeks, and then isolated primary mammary cells at different time points for MaSC quantification using an in vitro mammosphere formation and 3D-ECM sphere differentiation assay as well as the in vivo cleared mammary fat pad regeneration assay.