Isotope labelled α-Estradiol 3-Benzoate is a derivative of 17α-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.