Human prostatic acid phosphatase (also known as PAcP, PAP, PPAP, ACPP) is a prostate epithelium-specific differentiation antigen. The cellular form of PAcP functions as a neutral protein-tyrosine phosphatase, and is involved in regulating prostate cell growth. Specifically, PAcP catalyzes the conversion of orthophosphoric monoester to alcohol and orthophosphate. PAcP is synthesized under androgen regulation. The stimulated secretion of prostatic acid phosphatase is a hallmark of androgen action on human prostate epithelial cells, implicating PAcP as a useful tool in identifying atrophy of prostatic tissue. Cellular PAcP can down-regulate prostate cancer cell growth, at least partially by dephosphorylating c-ErbB-2/neu. Therefore, decreased cellular PAcP expression in cancer cells may be involved in prostate cancer progression. PAcP is the protein product of the human ACPP gene, which maps to chromosome 3q21-q23.