A ligand is any molecule that binds specifically to other one. There are many known and unknow ligands for both ERα and ERβ receptors. The structure of the unknown ligands don't need to resemble the structure of estradiol for the molecule to have the same effects, and therefore it is a time-consuming task to determine the effects a given compound has on the ER ,in this part I collect a brief description of these ligands. Estradiol and Estriol Estradiol is the agonist for the ER ( natural

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... ) -its structure is shown in last page. Estradiol is synthesized from cholesterol and secreted by the ovaries and cholesterol source can be food or biosynthesis. It is present in women and in men, in which case it is secreted from their testes. Estradiol is the main female sex hormone and it is essential for growth and reproductive endocrinoligy . Whnen it has an over-production -or an exposure to estrogen-like compoundsthere is a high risk of devoloping breast and endometrial cancer. When women reach menopause their ability to produce estradiol decreases and it can help to develop osteoporosis because estradiol is essential for bone growth and maintainance also Estradiol decreases the concentration of low-density lipoprotein (LDL) and increase the concentration of high density lipoprotein (HDL) and because HDL removes excess cholesterol from blood and tissue, women have a lower rate of coronary artery disease (CAD) than men before 50 years old. Estriol is a natural estrogen, which is produced during pregnancy. It is the major estrogen produced in the normal human fetus. There are indications that estriol maybe less carcinogenic than estradiol. Research has shown that estriol does not induce endometrial growth to the extent of the other estrogens, even at doses where estriol is effective for the relief of postmenopausal symptoms. The structure of estriol is also shown in last page.