Cystic fibrosis is a common, fatal disorder caused by abnormalities in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. CFTR encodes a chloride channel that regulates secretion in many exocrine tissues. The presentation of cystic fibrosis is highly variable as measured by the age of onset of disease, the presence of pancreatic insufficiency, or the progression of lung disease. Over 400 mutations in the CFTR gene have been described in cystic fibrosis patients and considerable effort has focused on the correlation between specific mutations and genotypes and clinical characteristics. Individual tissues display variation in their sensitivity to CFTR mutations. The vas deferens is functionally disrupted in nearly all males, whereas mild and severe pancreatic involvement is determined by the patient's genotype. The severity of pulmonary disease is poorly correlated with genotype, suggesting that there are other important genetic and/or environmental factors that contribute to lung infections and the subsequent disruption of lung function.