Cystic fibrosis: A common grave genetic disease that affects the exocrine glands and is characterized by the production of abnormal secretions, leading to mucus buildup that impairs the pancreas and, secondarily, the intestine. Mucus buildup in lungs can impair respiration. Abbreviated CF. Without treatment, CF results in death for 95 percent of affected children before age 5; however, a few long-lived CF patients have survived past age 60. Early diagnosis is of great importance. Treatment includes physical therapy to loosen the mucus in the lungs and use of pancreatic enzymes and medications to fight dangerous infections of the lungs. One in 400 couples is at risk for having children with CF. CF is a recessive trait, so the chance of an at-risk couple having a child with CF is 25 percent with each pregnancy. CF is caused by mutations in the CFTR (cystic fibrosis conductance regulator) gene, which is located on chromosome 7.