Synonym(s): Steroid 5-alpha-reductase3-oxo-5-alpha-steroid 4-dehydrogenases, catalyse the conversion of 3-oxo-5-alpha-steroid + acceptor to 3-oxo-delta(4)-steroid + reduced acceptor. The steroid 5-alpha-reductase enzyme is responsible for the formation of dihydrotestosterone, this hormone promotes the differentiation of male external genitalia and the prostate during foetal development. In humans mutations in this enzyme can cause a form of male pseudohermaphorditism in which the external genitalia and prostate fail to develop normally.