DPP4 and severe acute respiratory syndrome: For the three cell entry-related genes [ACE2 as the receptor of SARS-CoV and SARS-CoV-2 (Li et al., 2003; Hoffmann et al., 2020), DPP4 as the receptor of MERS-CoV (Raj et al., 2013), and protease TMPRSS2 for S protein priming of SARS-CoV-2 (Carrasco Pro et al., 2018)], we observed the dramatic changes in TMPRSS2 expression with very early induction during SARS-CoV-2 infection and the slightly down-regulated expression of ACE2 in cells infected with SARS-CoV-2 and SARS-CoV, whereas DPP4 was more up-regulated in MERS-CoV (Figure 4).