Further studies showed that inhibitors of cathepsin L blocked infection by both SARS-CoV and pseudotype expressing the SARS-CoV S protein, while not affecting the infection by either HCoV-NL63 or a retrovirus pseudotyped with the HCoV-NL63 S protein in HEK293T cells expressing Cathepsins and ACE2. This evidence concerns the gene PROS1 and infection.