Next, when we used 3-isobutyl-1-methylxantine (IBMX) and forskolin (Fsk) to activate cAMP-dependent CFTR-mediated Cl− secretion [17], [18] (Isc-IBMX/Fsk), we observed lumen-negative responses in tissues from individuals in both the “non-CF control” group (Fig.1A) and “Non-Classic CF” sub-group (Fig.1B) but lumen-positive responses for those in the “Classic CF” sub-group (Fig.1C). Here, CFTR is linked to cystic fibrosis.