P504S, also known as α-methylacyl coenzyme A racemase (AMACR), is a peroxisomal and mitochondrial enzyme that has been shown to play a role in bile acid synthesis and β-oxidation of branched chain fatty acids. In immunohistochemistry studies, P504S has been shown to be a specific marker of prostatic adenocarcinoma. Additionally, prostate glands involved in PIN have been found to express P504S, whereas P504S antibody was nearly undetectable in benign glands. P504S antibody has also been shown to stain many other types of carcinoma such as hepatoma, breast carcinoma, pancreatic islet tumor and desmoplastic small round cell tumor. HMW CK and p63 may serve as a useful panel with P504S.