Because the electrochemical gradient of chloride is the largest osmotic driver of fluid secretion by colonic epithelial cells, a defect in half of expressed CFTR channels, which are the main means of apical Cl− secretion, has been shown to reduce the secretory capability of the colon, and in turn reduce CF carriers’ susceptibility to secretory diarrhea as a result of cholera and E. coli toxins [30,60]. This evidence concerns the gene CFTR and cystic fibrosis.