Cathepsin S is a lysosomal enzyme that belongs to the papain family of cysteine proteases. This protein is expressed by antigen presenting cells including macrophages, B-lymphocytes, dendritic cells and microglia. Moreover, cathepsin S is expressed in some epithelial cells. Compared with the abundant cathepsins B, L and H, cathepsin S shows a restricted tissue distribution, with highest levels in spleen, heart, and lung. In addition, evidences indicated that cathepsin S generates A beta from amyloidogenic fragments of beta APP in the endosomal/lysosomal compartment, and is implicated in the pathogenesis of Alzheimer’s disease (AD) and Down Syndrome (DS).