DPP4 and infection: Enveloped viruses; case fatality rate: 30% (MERS-CoV), 10% (SARS-CoV), 3% (SARS-CoV-2); receptor: ACE2 (SARS-CoV, SARS-CoV-2); DPP4 (MERS-CoV); S protein proteolytic cleavage by cathepsins or TMPRSS2 is necessary for infection; RNA proofreading is viable due to the exoribonuclease activity