Prostein, also known as P501S, is a type IIIa plasma membrane protein which has been shown to be exclusively expressed in cells of normal and malignant prostate. Prostein is localized to the Golgi complex in the cytoplasm of cells and is expressed by both benign and neoplastic prostate tissue, whereas it has not been detected in any other normal or malignant tissue examined. It is a useful aid for classification of prostate adenocarcinomas.