Prostatic Acid Phosphatase (PSAP or PAP) is a member of the histidine acid phosphatase family. It is a non-specific phosphatase that is capable of dephosphorylating tyrosine residues as well as phospholipids under mildly acidic conditions. Prostatic specific acid phosphatase (PSAP) is involved in prostate epithelial differentiation. It is highly expressed in prostate, primary and metastatic carcinoma of the prostate. This marker may be helpful in pinpointing the site of origin in cases of metastatic carcinoma of the prostate, and is considered a more sensitive marker than PSA.