In rectal tissues from CF patients with two severe CFTR mutations and no detectable cAMP-mediated Cl− secretion (CFabsent) (Table 1), 1-EBIO failed to induce Cl− secretion (ΔIsc' = −0.7±0.8 μA/cm2, n = 7, P = 0.3) and had no effect on CCH-induced Isc' (Fig. 1 B,C,E). Here, CFTR is linked to cystic fibrosis.