Cystic fibrosis is caused by a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, commonly involving the loss of a single amino acid, phenylalanine. This loss changes the primary structure of the CFTR protein, preventing it from functioning properly to transport salt and water across membranes. The resulting defect leads to the production of abnormally thick and sticky mucus that clogs the lungs and obstructs the pancreas, preventing natural enzymes from helping the body break down food and absorb vital nutrients. To develop cystic fibrosis, children must inherit the mutated gene from both parents; inheriting it from only one parent results in an asymptomatic carrier.