CTSS and Alzheimer disease: In normal person, cystatins bind to cathepsins and prevent amyloid formation; however in AD, the level of cystatin goes down and the level of cathepsins and AChE enzyme increase which leads to decreases in acetylcholine and leads to formation Aβ peptides, donepezil when supplemented it binds to AChE and prevents acetylcholine breakdown thus increases the level of Acetyl choline which is required for normal functioning of the brain [16].