Spatial arrangement of target residues on opposite sides of CFTR’s composite site 1 in homology models. In a homology model of CFTR’s NBD dimer (Mornon et al., 2008), T460 (red) in the Walker A motif of NBD1 is close to NBD2 residues H1348 (gold; in the signature sequence), L1353 (blue; at the end of the signature sequence), and H1375 (magenta; in the D-loop).