ACE2 and severe acute respiratory syndrome: At the time of the SARS outbreak, SARS-CoV strains of a high degree of similarity were identified and isolated from patients and animals from markets; these strains’ S1-CTDs differed in receptor-binding motif (RBM) region by two residues only-Thr487 and Asn479 (humans) and Ser487 and Lys479 (civet viral strain) both of which recognize zinc peptide angiotensin-converting enzyme 2 (ACE2) however, the binding affinity of the human SARS-CoV S1-CTD for ACE2 is higher.