Cystic fibrosis (CF) is a genetic (inherited) disease that affects mainly the lungs, the pancreas, and the sweat glands. Some CF patients develop serious liver disease. The abnormal CF gene changes the makeup of mucus in the airways, making it thick and sticky. The body cannot clear this thick mucus from the airways, leading to recurrent lung infections that are difficult to treat. Thick secretions also block other glands, causing them to function poorly. About 30 000 persons in the United States have CF and more than 10 million carry the gene for CF. Carriers of the gene do not have symptoms of CF, but if they have a child who inherits 2 copies (one from each biological parent) of the CF gene, that child will develop cystic fibrosis and can also pass the CF gene on to any children he or she may have. In the past, most persons with CF found out about their condition when they had symptoms in childhood. Newborn screening programs can now lead to earlier diagnosis and therapy to limit the impact of the disease. The September 9, 2009, issue of JAMA includes an article about genetic variations that contribute to the development of serious liver disease in cystic fibrosis.