Estrogen receptors are a group of proteins found inside cells. They are receptors that are activated by the hormone estrogen (17β-estradiol). Two classes of estrogen receptor exist: ER, which is a member of the nuclear hormone family of intracellular receptors, and GPER (GPR30), which is a member of the rhodopsin-like family of G protein-coupled receptors. The ER's helix 12 domain plays a crucial role in determining interactions with coactivators and corepressors and, therefore, the respective agonist or antagonist effect of the ligand. Different ligands may differ in their affinity for alpha and beta isoforms of the estrogen receptor: estradiol binds equally well to both receptors, estrone, and raloxifene bind preferentially to the alpha receptor, estriol, and genistein to the beta receptor. Estrogen and its receptors are essential for sexual development and reproductive function, but also play a role in other tissues such as bone. Estrogen receptors are also involved in pathological processes including breast cancer, endometrial cancer, and osteoporosis. Alternative promoter usage and alternative splicing result in dozens of transcript variants, but the full-length nature of many of these variants has not been determined.