Cystatins are natural inhibitors of papain-like (family C1) and legumain-related (family C13) cysteine peptidases. The mammalian cystatin superfamily members are of three major types, including the type 1 cystatins (stefins), type 2 cystatins and the kininogens. As a member of type 2 cystatin, cystatin D is a single-domain protein and also has cysteine residues that form disulfide bridges. In contrast with the wider distribution of all the other family 2 cystatins, cystatin D is tissue-restricted expressed and has been found only in saliva and tears. and meanwhile, it displays an inhibition profile with a preferential inhibition on cathepsin S, H, L. Although the exact functions are largely unknown, it has reported that cystatin D is involved in the inhibition of virus replication and apoptosis.