CTSS and Alzheimer disease: At the same time, the current understanding of cathepsin function impairment in AD monocytes remains incomplete and is a subject of ongoing debate: some studies have reported no significant differences in the expression of cathepsins S, D, B, and L between AD patients and healthy individuals [85,97], while others, such as the study by Tian et al., have found that cathepsin D expression is reduced in AD monocytes compared to normal subjects [98].