Guanylyl cyclase C (GUCY2C) is a transmembrane receptor expressed primarily in the intestine that regulates chloride secretion via the cystic fibrosis transmembrane conductance regulator. Binding of GUCY2C to either the endogenous peptide guanylin or the bacterially derived heat-stable enterotoxin STa, results in increased levels of cGMP and the stimulation of water and chloride secretion. In the case of exposure to STa, this leads to debilitating secretory diarrhea.