MOC-31 is a glycoprotein on the cell-membrane (epithelial glycoprotein 2/epithelial cell adhesion molecule – Ep-CAM) that is widely distributed on epithelial cells and tumor cells.  MOC-31 is often used to differentiate adenocarcinoma (93% positive) from mesothelioma (93% negative).  Other tumors also typically negative for MOC-31 include:  hepatocellular carcinoma (HCC), germ cell tumors, and renal cell (some report up to 50%+) carcinomas.  Some tumors may have characteristic staining patterns (e.g. membraneous vs. cytoplasmic or apical).