TMPRSS2 and infection: The interaction with ACE2 receptors is mediated by RBD located on S1 domain (Figure 1).28 It has been demonstrated in an in vitro study that SARS-CoV-2 is unable to infect ACE2 knockout Vero E6 and Hela cells, suggesting its importance in viral entry.28,29 The cleavage of ACE2 and S-proteins for the entry of virus is mediated by TMPRSS2 enzyme, via membrane fusion, which is a major factor in the infection of COVID-19.