Adapted from original article at Wikipedia
5-alpha-reductase deficiency (5-ARD) is a condition caused by a mutation of the 5-alpha reductase type 2 gene. This gene encodes an enzyme that converts testosterone to dihydrotestosterone (DHT). DHT is necessary for the development of male genitalia in utero, and the resulting DHT deficency results in ambiguous external genitalia at birth. The condition affects only chromosomal males (i.e., those with XY chromosomes). Individuals with 5-ARD lack a uterus and Fallopian tubes (due to the normal action of Mullerian inhibiting factor), and possess testicles and Wolffian structures. Their external genitalia, however, can vary from normal male external genitalia, to ambiguous genitalia, to normal female genitalia (although with a tendency towards an enlarged clitoris). In the later cases the Wolffian ducts terminate in the perineum or in a pseudovagina. 5-ARD constitutes a variety of intersexualism.
Individuals with 5-ARD have XY chromosomes and testicles, and tend to have a vagina and labia, but with a small penis capable of ejaculation instead of a clitoris (this penis, however, appears to be a clitoris at birth). These individuals are normally raised as girls. However, come puberty, their testes will descend, their voice will deepen and they often will develop a male sexual identity. However, they develop only limited facial hair, and will not experience male-pattern baldness.