Studies on the relationship between CFTR function detected by ICM, CFTR genotype and CF phenotype demonstrated that patients in this group typically carry at least one ‘mild’ CFTR mutation (class IV and V) and that on average, residual CFTR function was associated with a milder form of CF characterized by sweat chloride concentrations in the lower diagnostic or intermediate range, long-term exocrine pancreatic sufficiency, later age at diagnosis, and less severe impairment in nutritional outcomes and lung function [28,51]. This evidence concerns the gene CFTR and cystic fibrosis.