(4-Methyl-d3)-17α-Estradiol is deuterium labeled 4-Methyl-17α-Estradiol (M304660). 4-Methyl-17α-Estradiol is a related compound of 17 α-Estradiol (E887995), a biologically active estrogen in human breast cancer cells in tissue culture. 17-Εstradiol 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 Alzheimer’s disease and ischemic stroke.