TMPRSS2 and infection: It has been reported that efficient infection of SARS-CoV and SARS-CoV-2 requires sequential cleavage of S by furin at the S1/S2 site and then by TMPRSS2 at the S2’ site or by CTSL at two specific sites [51] in endosome when TMPRSS2 expression is repressed (Fig 6A).