Cystic fibrosis (CF) patients without detectable disease-causing mutations in CFTR present a clinical phenotype very similar to, but possibly discrete from, classic CF. We hypothesize that the phenotype of these atypical CF patients is not due only to mutations in the carbonic anhydrase gene CA12 or in the genes encoding the epithelium sodium ion channel ENaC, but is due to mutations in novel genes that have yet to be associated with CF. The goal of this study is to determine the molecular etiology of this atypical CF.