Such defects have been described in several disease-associated mutations in membrane proteins, such as cystic fibrosis transmembrane conductance regulator (CFTR) in cystic fibrosis and bile salt export pump (BSEP) in progressive familial intrahepatic cholestasis type 2 [47, 48]. The gene discussed is CFTR; the disease is progressive familial intrahepatic cholestasis type 2.