The SRD5A1 genes encodes for 5-alpha-reductase which is an enzyme that converts testosterone to the more potent androgen, dihydrotestosterone (DHT). Common SNPs in the SRD5A genes predict for sensitivity to hormone replacement therapy. SRD5A1 is expressed most highly in nongenital skin (hair follicles), liver, and certain brain regions and at lower levels in prostate, epididymis, seminal vesicles, genital skin, testis, adrenal, and kidney. (Source 1)