CST4, also known as Cystatin-S, belongs to the cystatin family. It is an S-type cystatin, based on its high level of expression in saliva, tears and seminal plasma. The specific role in these fluids is unclear but antibacterial and antiviral activity is present, consistent with a protective function. This protein is 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. Recombinant human CST4, fused to His-tag at C-terminus, was expressed in insect cell and purified by using conventional chromatography techniques.