<unk>  or<unk>  is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous<unk> s that have<unk> ic hormonal activity: estrone (E1), estradiol <unk> , and estriol (E3). Estradiol, an estrane, is the most potent and prevalent. Another<unk>  called estetrol (E4) is produced only during pregnancy.<unk> s are synthesized in all vertebrates and some insects. Their presence in both vertebrates and insects suggests that<unk> ic sex hormones have an ancient evolutionary history. Quantitatively,<unk> s circulate at lower levels than androgens in both men and women. While<unk>  levels are significantly lower in males than in females,<unk> s nevertheless have important physiological roles in males. Like all steroid hormones,<unk> s readily diffuse across the cell membrane. Once inside the cell, they bind to and activate<unk>  receptors (ERs) which in turn modulate the expression of many genes. Additionally,<unk> s bind to and activate rapid-signaling membrane<unk>  receptors (mERs), such as GPER (GPR30). In addition to their role as natural hormones,<unk> s are used as medications, for instance in menopausal hormone therapy, hormonal birth control and feminizing hormone therapy for transgender women and nonbinary people.
Estrogen
Estradiol 
Estradiol
Androgen
Sex hormone
Progesterone
Progesterone 
Estriol