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Estrogen is the primary female sex hormone. It is responsible for it's regulation of the female reproductive system and secondary sex characteristic development. There are three major estrogens produced in the female endocrine system: estrone (E1), estradiol (E2), and estriol (E3); estetrol (E4) is only produced during pregnancy and plays no significant role in transgender hormone replacement therapy. The primary estrogen used in feminizing hormone therapy is estradiol.