The five general classifications of CFTR mutations are shown within the context of a single epithelial cell. The corresponding protein electrophoresis pattern for each class is depicted in the box to the right of each mutation class. A, B, and C refer to the typical mobilities of nascent CFTR (A, 130 kDa), core glycosylated CFTR (B, 150 kDa), and fully glycosylated CFTR (C, 180 kDa). Null refers to the absence of CFTR protein. CFTR, cystic fibrosis transmembrane conductance regulator.