CFTR and cystic fibrosis: Whereas CFTR‐mediated Cl− secretion serves to move mucus and Paneth cell‐derived defensins out of the intestinal crypts, studies in CF animal models revealed that CFTR‐dependent HCO3− secretion is required for the unfolding and release of mucins from neighbouring goblet cells (stained negatively for CFTR; Fig. 3A,E) and to prevent the formation of viscous and sticky mucus [45].