Cystic fibrosis (CF) is an inherited disease caused by a change (mutation) in the cystic fibrosis transmembrane regulator (CFTR) gene. It is a chronic, progressive disease that causes mucus to become thick and sticky. The mucus builds up and clogs passages in many of the body's organs, but mostly in the lungs and the pancreas . In the lungs, the mucus can cause serious breathing problems and lung disease. In the pancreas, the mucus can cause digestive problems and malnutrition, which can lead to problems with growth and development.