ESR1 and breast cancer: ToxCast and Tox21 contain several assays intended to evaluate chemicals’ effects on steroid hormone signaling pathways, including a diverse group of ER end points (ERα and ERβ hetero- and homodimerization, binding, transcriptional activity, and proliferation in an estrogen-dependent breast cancer cell line) (Judson et al. 2010; Rotroff et al. 2013b; Sipes et al. 2013; Tice et al. 2013).