Our data here show that CFTR-mediated Cl− secretion is absent or present at almost undetectable levels (<5%) in 244 biopsies from patients with classic forms of CF (Fig.1, Table S1), as defined by consensus criteria [7], [8], [10] including: early age at diagnosis (2.7±0.6 yrs), very high sweat-Cl− (114.23±2.33 mmol/l) and PI (9.08±2.04 μg/g) (Fig.2, 3, S3, S4, Table S3). The gene discussed is CFTR; the disease is cystic fibrosis.