Cross references:   Estrogen Receptor     Steroids   Intracellular Receptors  
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Estrogen - Wikipedia  
    "Estrogen or oestrogen (see spelling differences) is the primary female sex hormone and is responsible for development and regulation of the female reproductive system and secondary sex characteristics. Estrogen may also refer to any substance, natural or synthetic, that mimics the effects of the natural hormone.[1] The steroid 17β-estradiol is the most potent and prevalent endogenous estrogen, although several metabolites of estradiol also have estrogenic hormonal activity. Synthetic estrogens are used as part of some oral contraceptives, in estrogen replacement therapy for postmenopausal women, and in hormone replacement therapy for trans women.

The name estrogen comes from the Greek οἶστρος (oistros), literally meaning "verve or inspiration" but figuratively sexual passion or desire,[2] and the suffix -gen, meaning "producer of".

Estrogens are synthesized in all vertebrates[3] as well as some insects.[4] Their presence in both vertebrates and insects suggests that estrogenic sex hormones have an ancient evolutionary history. The three major naturally occurring forms of estrogen in women are estrone (E1), estradiol (E2), and estriol (E3). Another type of estrogen called estetrol (E4) is produced only during pregnancy. Quantitatively, estrogens circulate at lower levels than androgens in both men and women.[5] While estrogen levels are significantly lower in males compared to females, estrogens nevertheless also have important physiological roles in males.[6]

Like all steroid hormones, estrogens readily diffuse across the cell membrane. Once inside the cell, they bind to and activate estrogen receptors (ERs) which in turn modulate the expression of many genes.[7] Additionally, estrogens bind to and activate rapid-signaling membrane estrogen receptors (mERs),[8][9] such as GPER (GPR30).[10]