Cystic fibrosis is a genetic disease caused by a mutation in the CFTR (cystic fibrosis transmembrane conductance regulator) gene. This genetic change affects the structure and function of a protein that regulates salt movement between the inside and outside of cells. As a result, abnormally thick and sticky mucus accumulates in many organs, particularly the lungs and pancreas. This mucus buildup specifically obstructs the airways, leading to severe respiratory problems and other complications.