Cystic Fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene resulting in impaired chloride transport across epithelial cells. While many organs are involved, infection, inflammation and destruction of the lungs ultimately result in morbidity and mortality. There is an association between residual CFTR function and severity of disease, however there is great variability within specific mutations suggesting gene modifiers. Even though there are over 900 mutations in CFTR that are related to CF lung disease, F508 the most common one is represented in 70 percent of the American CF population. Thus, establishing a phenotype/genotype correlation using homozygote F508 patients is likely to identify genes that are responsible for a mild form of disease. Why is this important? Whereas since the identification of the gene CFTR a significant amount of knowledge has been accumulated on CFTR function and CF pathogenesis, the cure for CF (treated as a monogenic disease) has been elusive. Identification of genetic modifiers (that may explain why 10 percent of CF patients died before the age of 10, 1/3 before the age of 20 while 50 percent live over 32 years of age) should expand the therapeutic targets that may lead to shifting of the severe phenotypes to milder ones. Moreover, the approach outlined in this study may also result in a better understanding of CFTR and delta F508 biogenesis and function, as it may identify genes directly related to CFTR.