Although IFN-α, IFN-β, and IFN-γ treatment-induced ACE2 expression in tracheal cells, only IFN-α and IFN-β altered ACE2 expression in small airway cells [51], [80], [81], suggesting that ACE2 expression can be plausibly induced by either viral infections or interferon treatment [51]. Here, ACE2 is linked to viral infectious disease.