CTSL and infection: For example, SARS-CoV enters into the cell mainly via the endosomal membrane fusion pathway where its S protein is cleaved by endosomal cathepsin L and activated.25 Inducing the S1/S2 furin-recognition site could significantly increase the capacity of SARS-CoV S protein to mediate cellular membrane surface infection.26 Interestingly, SARS-CoV-2 harbors the S1/S2 cleavage site in its S protein, but its specific role in S protein-mediated membrane fusion and viral life-cycle remains to be further explored (Supplementary information, Fig. S7).