In most androgen target tissues, the first step of androgen action is the 5 alpha-reduction of testosterone to DHT which binds to the androgen receptor with an affinity 3 to 4 fold higher than testosterone. Two genes, encoding two isozymes of 5 alpha-reductase (5 alpha-R) have been cloned. The two isoforms, 5 alpha-R1 and 5 alpha-R 2 are located on chromosomes 5 and 2 respectively and differ in optimal pH, substrate and inhibitor affinities and tissue expression. 5 alpha-R 2 is responsible for sexual differentiation. It is the major form expressed in the prostate where it seems necessary for embryonic growth and development. 5 alpha-reductase deficiency results in androgen insensitivity due to abnormal 5 alpha-R 2. Affected patients are XY individuals with a very peculiar form of male pseudohermaphroditism: they have feminine genitalia at birth and masculinize at puberty. 29 mutations, spanning the whole coding portion of the gene, have been described; correlation between mutations and enzyme activity have led to the suggestion that both the N- and the C-terminal end of the gene are involved in substrate binding, whereas the cofactor binding-site is located in the C-terminus. In contrast to androgen insensitivity due to 5 alpha-reductase deficiency, increased 5 alpha-reductase activity can result in androgen hypersensitivity as described in idiopathic hirsutism or benign prostatic hyperplasia. In these case 5 alpha-R 1 could possibly be involved.