CFTR and cystic fibrosis: For example, in cystic fibrosis (CF) patients homozygous for the ΔF508 mutation (deletion of a phenylalanine at site 508) in the CFTR gene, the misfolded protein in endoplasmic reticulum (ER) is mostly degraded after failing to go through the quality control system (Fraser-Pitt and O'Neil 2015).