REPRODUCTIVE ENDOCRINOLOGY: Edited by Wendy Kuohung5-Alpha reductase deficiency a 40-year retrospective reviewOkeigwe, Ijeoma; Kuohung, WendyAuthor Information Department of Obstetrics and Gynecology, Boston University Medical Center, Boston, Massachusetts, USA Correspondence to Ijeoma Okeigwe, MD, MPH, Department of Obstetrics and Gynecology, Boston University Medical Center, 85 East Concord Street, 6th Floor, Boston, MA 02118, USA. Tel: +1 617 414 5167; fax: +1 617 414 7300; e-mail: [email protected] Current Opinion in Endocrinology & Diabetes and Obesity: December 2014 - Volume 21 - Issue 6 - p 483-487 doi: 10.1097/MED.0000000000000116 Buy Metrics Abstract Purpose of review 5-Alpha reductase is an enzyme responsible for the conversion of testosterone to dihydrotestosterone. This key enzyme is responsible for triggering masculinization of the male external genitalia. Discovery of 5-alpha reductase deficiency as a syndrome of disordered male sexual development led to our molecular understanding of the role that this key enzyme plays in male sexual differentiation. This article will review the clinical and molecular history behind the discovery of 5-alpha reductase deficiency. Recent findings Three different genes encoding for 5-alpha reductase have been identified, with 5-alpha reductase type 2 being implicated in disordered male sexual development. Summary The discovery of 5-alpha reductase deficiency has not only shed light on the crucial role of 5-alpha reductase, testosterone, and dihydrotestosterone in male sexual differentiation but it also has facilitated the discovery of novel therapeutic applications of 5-alpha reductase inhibitors in clinical practice. Copyright © 2014 Wolters Kluwer Health, Inc. All rights reserved.