After infection of SARS-CoV-2, the interferon-stimulated gene (ISG) 56 RNA level in Calu-3 cells is significantly higher than that in HEK293-ACE2 cells (Fig. 4E), which indicates that SARS-CoV-2 infection triggering the expression of IFN in Calu-3 cells is stronger than that in HEK293-ACE2 cells. Here, ACE2 is linked to infection.