CFTR and cystic fibrosis: Variants that cause a disruption of the conductivity of the CFTR channel (Class 4), the formation of a reduced amount of CFTR protein (Class 5), or the formation of an unstable protein that is prematurely recycled from the apical membrane and degrades in lysosomes (Class 6) will lead to a partial decrease in the functional activity of the CFTR channel, which is often associated with less severe CF phenotypes [7].