Lung epithelial cells have a thin layer of watery mucus on their surface. The normal allele of the CFTR gene codes for a transport protein that transports chloride ions out of epithelial cells. Fig. 4.1 is a diagram of part of the cell surface membrane and the mucus layer of an epithelial cell with normal CFTR proteins. [Figure 4.1] Cystic fibrosis (CF) is a genetic disorder caused by having two recessive alleles of CFTR. In severe cases of CF, the transport proteins are not added to the cell surface membrane. This causes the mucus layer to be thick and sticky.