Prostate Acid Phosphatase. Prostatic Differentiation Markers. PAP is a marker of prostatic differentiation and is more sensitive (but less specific) than PSA. It is expressed in normal, hyperplastic, and neoplastic prostatic epithelium. When coupled with PSA, it can be of great help in ascertaining prostatic epithelial origin of a given neoplasm. Occasionally, carcinoid rumors and bladder carcinomas can be positive for PAP.