Normal and abnormal CFTR proteins. Computer illustration of a normally functioning cystic fibrosis transmembrane conductance regulator (CFTR) protein (left) and a malfunctioning mutant CFTR protein (right) in a cell membrane (red). Carbohydrate is yellow, chloride is green, phosphate is turquoise and adenosine triphosphate (ATP) is purple. CFTR is an ion-channel that moves chloride and thiocyanate ions across epithelial cell membranes. Functional irregularities of these proteins, caused by mutations of the CFTR gene, lead to malfunctioning of epithelial fluid transport, causing mucous (beige, upper right) to build up outside the cells in the lung, pancreas and other organs, resulting in cystic fibrosis.