PROS1 and infection: Strong luciferase activity detected in the MDCK cells inoculated with FLuc-SARS-S pseudovirus indicated viral entry mediated by the binding between SARS-CoV S protein and ACE2 on the surface of MDCK cells without interference from other cellular factors, such as glycosylation on S protein [34] or ACE2 [35], which can cause MDCK cells to resist the infection by live SARS-CoV [31].