Third, why do coronaviruses rely on peptidases as theDPP4. They are all recognized by S1-CTDs of different coronaviruses. It is highly unlikely that the use of peptidases as coronavirus receptors peptidase activities have no effects on coronavirus entry, indicating that their common physiological function in degrading peptides was not the reason why they were selected as coronavirus receptors. To fully understand why peptidases became chosen receptors for coronaviruses, it will be important in the future comprehensively  examine the physiological functions of these peptidase receptors.