CFTR and cystic fibrosis: Recently, these variants have been categorized into 7 classes based on CFTR protein dysfunction and/or gene expression [12] (Fig. 1): Class I are protein production variants that result in no functional CFTR protein with roughly 22% of CF patients harboring at least 1 mutant allele; Class II are protein processing variants that create misfolded CFTR protein and reduced expression on the cell membrane to function.