Cystic fibrosis is caused by a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The resulting protein is not able to regulate the movement of salt in and out of cells as well, causing thick mucus in the respiratory, digestive, and reproductive symptoms, and increased salt levels in sweat. The different ways a mutation can develop will affect the severity of the symptoms that present. Children need to inherit the mutation from both parents in order to have cystic fibrosis; if they only inherit from one parent, they will be asymptomatic carriers.