Estrone is an estrogen, specifically an agonist of the estrogen receptors ERα and ERβ.[1][4] It is a far less potent estrogen than is estradiol, and as such, is a relatively weak estrogen.[1][4][5] Given by subcutaneous injection in mice, estradiol is about 10-fold more potent than estrone and about 100-fold more potent than estriol.[6] According to one study, the relative binding affinities of estrone for the human ERα and ERβ were 4.0% and 3.5% of those estradiol, respectively, and the relative transactivational capacities of estrone at the ERα and ERβ were 2.6% and 4.3% of those of estradiol, respectively.[4] In accordance, the estrogenic activity of estrone has been reported to be approximately 4% of that of estradiol.[1] In addition to its low estrogenic potency, estrone, unlike estradiol and estriol, is not accumulated in estrogen target tissues.[1] Because estrone can be transformed into estradiol, most of the estrogenic potency of estrone in vivo is actually due to conversion into estradiol.[1] As such, estrone is considered to be a precursor or prohormone of estradiol.[3]