Caco2 (27 (link)) and Calu3 (28 (link)) cell lines were obtained from ATCC and grown in DMEM (Dulbecco's Modified Eagle's medium) with 10% FBS (fetal bovine serum). For all experiments with Caco2 cells they were harvested 48 h post-confluence, a time at which CFTR expression is close to maximum levels (29 (link)). Human bronchial epithelial (HBE) cells were donated by Dr Scott Randell (UNC) and cultured in Bronchial Epithelial Cell Growth Medium, Lonza. Adult human epididymis cells (caput) were described previously (30 (link)). Skin fibroblasts (GM08333) from Coriell Institute were grown in Ham's F-10 media supplement with 15% FBS.