CFTR and cystic fibrosis: These findings confirm the registry data based on culture-dependent diagnostics showing that the amount of CFTR activity is the key determinant for the microbial colonization of CF lungs (1, 71); i.e., PI patients carry CFTR mutation genotypes which confer null or minimal CFTR activity, whereas PS patients harbor CFTR genotypes which confer some residual CFTR activity, about 10 to 20% of wild type (51, 70, 72).