CFTR and cystic fibrosis: PI patients carry two severe disease-causing CFTR mutations in the CFTR gene that confer no or only minimal CFTR (cystic fibrosis transmembrane conductance regulator) function, whereas PS patients harbor at least one CFTR allele that confers residual CFTR activity in the range of 10 to 20% of the wild-type level and is typically associated with a milder course of CF disease (1, 51).