ACE2 and infection: Next, we employed a luciferase reporter SARS-CoV-2 S pseudotyped lentivirus system to study the effect of these cysteine cluster mutations on the cellular entry and infectivity of the S protein by measuring the luciferase activity 48 h after pseudovirus infection of HEK293T-ACE2 or Caco-2 cells (Figure 1C).