Estradiol (known as alpha Estradiol or 17 alpha Estradiol) is a biologically active estrogen in human breast cancer cells in tissue culture.17-estradiol and its selective receptor, ER-X, are not part of a classical hormone receptor endocrine system but of a system with important autocrine paracrine functions in the developing and adult brain. 17-Estradiol may have enormous implications for hormone replacement strategies at the menopause and in the treatment of such neurodegenerative disorders as Alzheimera?s disease and ischemic stroke.