The most common one, occurring in 70–90% of CF patients, is the deletion of phenylalanine 508 (ΔF508) in NBD1 that causes inappropriate folding and structural instability of ΔF508-CFTR that, for this reasons, remains trapped in the endoplasmic reticulum and is rapidly degraded by the ubiquitin/proteasome system. Here, CFTR is linked to cystic fibrosis.