<unk> (E4) is an estrogen medication and naturally occurring steroid hormone which is used in combination with a progestin in combined birth control pills and is under development for various other indications. These investigational uses include menopausal hormone therapy to treat symptoms such as vaginal atrophy, hot flashes, and bone loss and the treatment of breast cancer and prostate cancer. It is taken by mouth.<unk> is a naturally occurring and bioidentical estrogen, or an agonist of the estrogen receptor, the biological target of estrogens like endogenous estradiol. Due to its estrogenic activity,<unk> has antigonadotropic effects and can inhibit fertility and suppress sex hormone production and levels in both women and men.<unk> differs in various ways both from other natural estrogens like estradiol and synthetic estrogens like ethinylestradiol, with implications for tolerability and safety. For instance, it appears to have minimal estrogenic effects in the breasts and liver.<unk> interacts with nuclear ERα in a manner identical to that of the other estrogens and distinct from that observed with Selective Estrogen Receptor Modulators (SERMs).<unk> was first discovered in 1965, and basic research continued up until 1984. It started to be studied again as well as investigated for potential medical use in 2001, and by 2008, was of major interest for possible medical use. As of 2021,<unk> is in mid- to late-stage clinical development for a variety of indications.
Estetrol 
Gestodene
Hexestrol
Fosfestrol
Benzestrol
Moxestrol
Dimestrol
Dienestrol