Enzyme activity was not altered by the presence of either segmental duplication, we note neither of these models carries an additional copy of Cstb. These data indicate that three copies of Cstb is not sufficient to modulate cathepsin B activity in a mouse model of AD-DS, consistent with our previous findings from a segmental duplication model of Cstb [34], nor is an additional copy of Hsa21 homologues encoded on Mmu16 or Mmu17 sufficient to alter activity. The gene discussed is CSTB; the disease is Alzheimer disease.