Cystatin-S, also called Cystatin-SA-III or Salivary acidic protein, is usually expressed in submandibular and sublingual saliva but not in parotid saliva(at protein level). This protein, which is phosphorylated at both its N- and C-terminal regions, belongs to the cystatin family. Cystatin-S can function as an inhibitor to papain or ficin, it partially inhibits stem bromelain and bovine cathepsin C.