Estrone (E1), also spelled oestrone, is a steroid, a weak estrogen, and a minor female sex hormone.[1] It is one of three major endogenous estrogens, the others being estradiol and estriol.[1] Estrone, as well as the other estrogens, are synthesized from cholesterol and secreted mainly from the gonads, though they can also be formed from adrenal androgens in adipose tissue.[2] Relative to estradiol, both estrone and estriol have far weaker activity as estrogens.[1] Estrone can be converted into estradiol, and serves mainly as a precursor or metabolic intermediate of estradiol.[1][3]