5α-reductase

an enzyme that catalyzes the irreversible reduction of testosterone to dihydrotestosterone with NADPH as the hydrogen donor. Deficiency of the enzyme, with resultant deficiency in dihydrotestosterone, is an autosomal recessive condition characterized by male pseudohermaphroditism, prepubertal ambiguous or female-type external genitalia (usually some form of perineal hypospadias), and some masculinization at puberty without gynecomastia.