CFTR and cystic fibrosis: To evaluate the robustness of colonic CFTR-mediated Cl− secretion as a diagnosis/prognosis biomarker and thus help overcoming such difficulties, we assessed CFTR (dys)function ex vivo in 524 rectal biopsies from 118 individuals, including the largest cohort of CF patients ever analysed by this approach (n = 51), a non-CF (control) group (n = 18) and individuals with clinical CF suspicion to confirm/exclude a CF diagnosis (n = 49).