Fulvestrant is a 3-hydroxy steroid that is 17beta-estradiol in which the 7alpha hydrogen has been replaced by a nonyl group in which one of the hydrogens of the terminal methyl has been replaced by a (4,4,5,5,5-pentafluoropentyl)sulfinyl group. An estrogen receptor antagonist, it is used in the treatment of breast cancer. It has a role as an antineoplastic agent, an estrogen receptor antagonist and an estrogen antagonist. It is a 3-hydroxy steroid, a 17beta-hydroxy steroid, an organofluorine compound and a sulfoxide. It derives from a hydride of an estrane.