CFTR and cystic fibrosis: Since our previous results have established that quantification of rectal CFTR-mediated Cl− secretion is a sensitive test for the diagnosis and prognosis of CF disease [18], hereunder we applied it to the largest cohort of CF patients and highest number of rectal biopsies ever assessed in a single study analyzing CFTR-mediated Cl− secretion in native tissue ex vivo to evaluate its robustness as a diagnosis/prognosis biomarker.