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.