This protein strongly inhibits papain and ficin, partially inhibits stem bromelain and bovine cathepsin C, but does not inhibit porcine cathepsin B or clostripain. Papain is inhibited non-competitively. Expressed in submandibular and sublingual saliva but not in parotid saliva (at protein level). Expressed in saliva, tears, urine and seminal fluid. Phosphorylated at both its N- and C-terminal regions. Monophosphorylated at Ser-23, also called form S1. Diphosphorylated at Ser-21 and Ser-23, also called form S2. Belongs to the cystatin family.