The function of Ensembl gene identifier ENSG00000145545 (SRD5A1, steroid 5 alpha-reductase 1) is as follows. Catalyzes the irreversible stereospecific reduction of the delta 4,5 bond (double bond between carbons 4 and 5) of various 3-oxo steroids (3-keto steroids) producing their 5alpha dihydro-3-oxo forms (PubMed:2339109, PubMed:23685396). Converts testosterone into 5-alpha- dihydrotestosterone (DHT), a more potent androgen as it is the preferred ligand for androgen receptor (AR) transactivation, making this reaction a key step in male sexual differentiation during development (PubMed:2339109, PubMed:23685396). Besides testosterone, it can also act on other steroids, including progesterone, androstenedione, and corticoids, producing metabolites with diverse roles (PubMed:2339109). Hence, it plays a central role in sexual differentiation and androgen physiology (PubMed:2339109). .