ACE2 and infection: Further, when pINT2-EF1α-S-ΔRS-HA was cotransfected with pEGFP-C1, a construct expressing enhanced green fluorescent protein in a cell line stably expressing human ACE2 and TMPRSS2 (HEK/ACE2/TMPRRS2/Puro cells), large syncytia were observed (Figure 1B; Supplementary Figure 1A), recapitulating the syncytiogenic activity of SARS-CoV-2 S during infection and was ascribable to the interaction with ACE2 and TMPRSS2.