CFTR and cystic fibrosis: Overall, we observed a similar effect of extracellular phosphate on the response to forskolin and P5 in the C18-pre-treated F508del-CFTR-expressing FRT epithelia (Supplementary Fig. 1) and the C18-pre-treated CF hAEC epithelia (Supplementary Fig. 2), to that found with VX-809-pre-treated epithelia.