What is Estrogen? Estrogens or oestrogens (see spelling differences) are a group of compounds named for their importance in both menstrual and estrous reproductive cycles. They are the primary female sex hormones. Natural estrogens are steroid hormones while some synthetic ones are non-steroidal. The name comes from the Greek ' (oistros), literally meaning "verve or inspiration" but figuratively sexual passion or desire, and the suffix -gen, meaning "producer of". Estrogens are synthesized in all vertebrates as well as some insects. Their presence in both vertebrates and insects suggests that estrogenic sex hormones have an ancient evolutionary history. Estrogens are used as part of some oral contraceptives, in estrogen replacement therapy for postmenopausal women, and in hormone replacement therapy for trans women. Like all steroid hormones, estrogens readily diffuse across the cell membrane. Once inside the cell, they bind to and activate estrogen receptors which in turn modulate the expression of many genes. Additionally, estrogens bind to and activate rapid-signaling membrane estrogen receptors, such as GPER (GPR30).