The S1 domain acts as a major antigen on the surface of the virus[40] and has a receptor-binding domain (RBD).[25] The 18 residues of ACE-2 interact with the RBD (contain 14 amino acids) of SARS-CoV spike protein,[45] and for this contact, K341 of ACE-2 and R453 residue of RBD play the most important role.