Symptom-causing mutations interfere with the protein responsible for transporting chloride across the cell membrane. Based on the means by which they disrupt CFTR protein production or function, there are six classes of CF mutations, which are not mutually exclusive. Patients with class I and II mutations, which result in very limited or no CFTR protein production, are more likely to manifest typical CF symptoms, including pulmonary disease and pancreatic insufficiency, in infancy or early childhood. Patients with classes III, IV, V, and VI mutations have some protein production. Patients who have class IV mutations, in which protein production is normal, tend to have milder symptoms than patients with two class I or II mutations, even if they also have a single class I or II mutation.