CST4 is a member of the cystatin superfamily which contains proteins that hold multiple cystatin-like sequences. Several family members are active cysteine protease inhibitors， whereas others have lost or possibly never attained this inhibitory activity. CST4 strongly inhibits papain （non-competitively） and ficin， partially inhibits stem bromelain and bovine cathepsin C， however does not inhibit porcine cathepsin B or clostripain.
