Cystatin-4 (CST4) is also known as Cystatin-S, Salivary acidic protein 1, which is a secreted protein, which belongs to the cystatin family. Cystatin-S is expressed in submandibular and sublingual saliva but not in parotid saliva (at protein level) and is also expressed in saliva, tears, urine and seminal fluid. Cystatin-4 / Cystatin S 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.