TMPRSS2 and infection: Cells naturally expressing the ACE2 receptor and the transmembrane serine protease 2 (TMPRSS2) are then added to the plates and incubated for 48–72 h otherwise co-transfection of ACE2 and TMPRSS2 plasmids can be carried out to render cells such as HEK 293T, and HeLa permissive to infection.