Investigation of the functional determinants of S-mediated entry into target cells using a murine leukemia virus (MLV) psudotyping system. To assess the ability of SARS-CoV-2 S to promote entry into target cell, the investigation compared transduction  of SARS-CoV-2 S-MLV and SARS-CoV S-MLV into VeroE6 cells, that are known to express ACE2 and support SARS-CoV replication.