5-alpha reductase is an enzyme (EC 220.127.116.11) that converts testosterone, the male sex hormone, into dihydrotestosterone. There are two isoenzymes, steroid 5-alpha reductase 1 and 2 (SRD5A1 and SRD5A2). The second isoenzyme is deficient in 5-alpha-reductase deficiency, which leads to a form of intersexualism.
SRD5A1 is located on chromosome 5p15, while SRD5A2 is located on 2p23.
Finasteride inhibits the function of only one of the isoenzymes (type 1), while dutasteride inhibits both forms. These drugs are used in benign prostatic hyperplasia, prostate cancer and baldness (finasteride).