CFTR and cystic fibrosis: In contrast, misfolded mutant proteins cannot reach the cell surface and instead accumulate in the ER, promoting ER stress.9 CFTR is commonly mutated among patients suffering from cystic fibrosis, with F508 deletion (F508del), the most common hotspot mutation.119 CFTR is a 168 kDa protein and a member of the ATP-binding cassette transporter family, and its ATP-binding capacity allows it to facilitate the translocation of substrates across cellular membranes.