Malfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) leads to cystic fibrosis, but the regulation of CFTR is not fully understood.Here, we identified the intermediate filament protein keratin K18 (K18) as a CFTR binding protein by various approaches.We mapped a highly conserved "hydrophobic patch" (F1413LVI) in the CFTR carboxy-terminus, known to determine plasmalemmal CFTR stability, as the K18 binding site.On the other hand, the C-terminal tail of K18 was found to be a critical determinant for binding CFTR.