CST3 and Parkinson disease: Indeed, in our study, the expression of cystatin C protein was increased in PD samples and followed the same direction of expression as its gene, CST3. Cystatin C is an endogenous inhibitor of cysteine proteases such as cathepsins B, H, K, S, and L and is present in all mammalian body fluid and tissues (Bobek and Levine 1992).