CTSB and Alzheimer disease: To test the hypothesis that raised CSTB, caused by an additional copy of Hsa21 causes the changes to cathepsin B activity in the brains of individuals who have AD-DS, we measured CSTB and cathepsin B protein abundance, and cathepsin B activity in fibroblasts isolated from infants and children who have DS (trisomy 21) and matched disomic individuals (Table 2).