Equilin is an estrogen from horses with the chemical name 3-hydroxyestra-1,3,5,7-tetraen-17-one. Equilin is one of the estrogens present in the mixture of estrogens isolated from horse urine and marketed as Premarin. Premarin became the most commonly used form of estrogen for hormone replacement therapy in the United States of America. Estrone is the major estrogen in Premarin (about 50%) and equilin is present as about 25% of the total. Estrone is a major estrogen that is normally found in women. Equilin is not normally present in women, so there has been interest in the effects of equilin on the human body. The estrogens in Premarin are present mainly as "conjugates", modified chemical froms in which the active estrogen is coupled to another chemical group such as sulfate. Estrone sulfate is usually the major form of estrogen in women. After being taken into a woman's body, the conjugated estrogens of Premarin are converted to the active unconjugated estrogens or excreted from the woman's body. Estrone can be converted to estradiol, which is thought to be the major active estrogen in women.