 Several mutations in the "CFTR" gene can occur, and different mutations cause different defects in the CFTR protein, sometimes causing a milder or more severe disease.These protein defects are also targets for drugs which can sometimes restore their function.ΔF508-CFTR, which occurs in >90% of patients in the U.S., creates a protein that does not fold normally and is not appropriately transported to the cell membrane, resulting in its degradation.