4-Methoxyestriol
4-Methoxyestriol is an endogenous estrogen metabolite. It is the 4-methyl ether of 4-hydroxyestriol and a metabolite of estriol and 4-hydroxyestriol. 4-Methoxyestriol has very low affinities for the estrogen receptors. Its relative binding affinities for estrogen receptor alpha and estrogen receptor beta are both about 1% of those of estradiol. For comparison, estriol had RBAs of 11% and 35%, respectively.