Prostate-specific antigen (PSA) as a prostate cancer marker is a chymotrypsin-like serine protease which is expressed primarily by both normal prostate epithelium and the vast majority of prostate cancers. PSA expression is tightly regulated by androgen through the activation of androgen receptor. However, in the absence of androgens, PSA gene expression can become elevated. This suggests that either the AR can be activated in the absence of androgen to elevate PSA gene expression or that another transcription factor acting on the PSA promoter is stimulated. This article reviews the research on the structure of the PSA promoter and enhancer and the mechanisms of the PSA expression.