5α-Reductase 2 deficiency (5αR2D) is an autosomal recessive condition caused by a mutation in SRD5A2, a gene encoding the enzyme 5α-reductase type 2 (5αR2). The condition is rare, affects only genetic males, and has a broad spectrum. 5αR2 is expressed in specific tissues and catalyzes the transformation of testosterone (T) to 5α-dihydrotestosterone (DHT). DHT plays a key role in the process of sexual differentiation in the external genitalia and prostate during development of the male fetus. 5αR2D is a result of impaired 5αR2 activity resulting in decreased DHT levels. This defect results in a spectrum of phenotypes including overt genital ambiguity, hypospadias, and micropenis. Affected males still develop typical masculine features at puberty (deep voice, facial hair, muscle bulk) since most aspects of pubertal virilization are driven by testosterone, not DHT. Management of this condition in the context of sex assignment is a challenging and controversial area. Diagnostic availability, local laws, and parental anxiety all play roles in treatment decisions. The investigation of 5αR2D as a disease has played a key role in the biochemical characterization of the SRD5A2 gene, the 5αR2 enzyme, and DHT in male sexual differentiation. Presentation Mutations in the SRD5A2 gene can result in a 46,XY disorder of sex development (46,XY DSD) called 5α-reductase 2 deficiency (5αR2D). The mutations are inherited in an autosomal recessive pattern can be either homozygous or, less frequently, compound heterozygous loss-of-function. Affected males exhibit a broad spectrum of presentation including atypical genitalia (ranging from female appearing to undervirilized male), hypospadias, and isolated micropenis. The internal reproductive structures (vasa deferentia, seminal vesicles, epididymides and ejaculatory ducts) are normal but testes are usually undescended and prostate hypoplasia is common. Males with the same mutations in SRD5A2 can have different phenotypes suggesting additional factors that are involved in clinical presentation. Females with the same mutations in SRD5A2 as affected males (as seen in siblings) are unaffected and have normal female phenotypes and reproductive function. Virilization of genitalia with voice deepening, development of muscle mass occur at puberty in affected males and height is not impaired. Gynecomastia is uncommon and bone density is normal in contrast to 46,XY DSD from other causes such as partial androgen insensitivity syndrome and 17β-hydroxysteroid dehydrogenase 3 deficiency. Hair on the face and body is reduced and male pattern baldness does not occur. Mechanism 5α-Reductase type 2 (5αR2) is an enzyme, encoded by the SRD5A2 gene, that is expressed in