CTSD is a good candidate gene for AD [62–64], as it encodes a lysosomal protease important for the degradation of various substrates, including disease-associated proteins, such as α-synuclein (a-syn), amyloid precursor protein (APP) and tau, which tend to aggregate if not efficiently degraded in neurodegenerative disorders [65]. This evidence concerns the gene CTSD and Alzheimer disease.