A defect in the CFTR gene causes cystic fibrosis (CF). This gene makes a protein that controls the movement of salt and water in and out of your body's cells. In people who have CF, the gene makes a protein that doesn't work well. This causes thick, sticky mucus and very salty sweat.
Respiratory signs and symptoms. The thick and sticky mucus associated with cystic fibrosis clogs the tubes that carry air in and out of your lungs. This can cause: A persistent cough that produces thick spit (sputum) and mucus.