The fusion and entry process requires three categories of proteases that function at different stages of infection: (a) phase of viral attachment: proprotein convertases (e.g., furin), (b) phase of S1 cleavage and detachment from the S2 domain: cell surface proteases [e.g., type II transmembrane serine protease (TMPRSS2)] and (c) phase of intraviral endocytosis: lysosomal proteases (e.g., cathepsin B/L) (Zhang et al., 2021). Here, FURIN is linked to infection.