CTSD and Alzheimer disease: Equistatin has been shown to be a very potent inhibitor of papain and a specific inhibitor of the aspartic proteinase cathepsin D. While papain-like cysteine proteases have been implicated in various diseases of the central nervous system, such as brain tumors, Alzheimer’s disease, stroke, cerebral lesions, neurological autoimmune diseases and certain forms of epilepsy, aspartic proteinase cathepsin D is involved in the pathogenesis of breast cancer [5] and possibly Alzheimer’s disease [6].