CFTR and cystic fibrosis: Based on their molecular consequences the several hundred identified CF mutations have been categorized into classes, such as those that impair synthesis of the full-length CFTR polypeptide (Class I), processing and trafficking of the CFTR protein (Class II), channel gating (Class III), or anion permeation through the open pore (Class IV) (De Boeck and Amaral, 2016).