Steroid 5a-Reductase is an important enzyme in androgen physiology because it catalyzes the conversion of testosterone into the more potent 5a-dihydro-testosterone, which mediates androgen effects on target tissues. The enzyme exists as two isoforms: type 1, which is expressed mainly in the skin; and type 2, which is expressed mainly in the prostate. In cultured human skin cells, 5a-Reductase 1 shows heterogeneity of protein, and has different levels of transcriptional and translational expression. 5a-Reductase 1 is expressed in all portions of the hair follicle, whereas 5a-Reductase 2 is expressed only in mesenchymal portions. In addition, 5a-Reductase 1 is mainly expressed in human breast carcinoma and may play a role in the in situ production and actions of the potent androgen 5a-dihydrotestosterone, including inhibition of cancer cell proliferation in hormone-dependent human breast carcinoma. The 5a-Reductase-3a-hydroxysteroid dehydrogenase complex is present in the human brain, suggesting that the complex may be involved in the synthesis of neuroactive steroids or the catabolism of neurotoxic steroids.