The S protein of SARS does not have furin cleavage sites and is thus not cleaved during biogenesis in producing cells, but it can be cleaved during infection of target cells by multiple cellular proteases, such as endosomal cathepsin L, type II transmembrane serine proteases (TTSPs), human airway trypsin-like protease (HAT), or transmembrane protease, serine 2 (TMPRSS2), to facilitate efficient entry [32–35]. Here, PROS1 is linked to infection.