Guanylyl Cyclase C, one of a family of seven homologous proteins, is selectively expressed by intestinal epithelial cells, where it catalyzes the production of cGMP following binding of its paracrine hormones guanylin or uroguanylin. Also, GUCY2C is normally expressed by neurons of the hypothalamus and striatum in brain.

Of importance, GUCY2C is over-expressed by nearly all metastatic colorectal cancers, and a substantial subset of metastatic gastric, esophageal, and pancreatic cancers.

Limited expression in normal tissues and universal overexpression by primary and metastatic colorectal tumors, and a subset of other GI tumors, makes GUCY2C a useful molecular biomarker to detect metastatic cancers, and a potential chemotherapeutic and immunotherapeutic target in patients with those cancers.