CFTR and cystic fibrosis: Since then, Calu-3 has been extensively used to study drug permeation and transport mechanisms of small drug molecules and xenobiotics to be administered via the pulmonary route [95] The tight monolayer present in this cell line also expresses the cystic fibrosis transmembrane conductance regulator (CFTR) protein, which is essential for studies regarding cystic fibrosis.