Although RGD motifs are the most common mechanism of integrin binding, there are other cell surface proteins which bind integrins despite lacking an RGD motif. ADAM 9, for example, binds through a hypervariable loop stabilised by disulphide bridges, which protrudes from the surface of the protein structure [43, 44] . The cellular adhesion molecule ICAM-1 is hypothesised to bind via immunoglobulin type domains [45] , whereas other proteins have been predicted to bind through a (D/E)ECD motif [46] . Neither ACE nor ACE2 contain an ECD motif.