In this sub-set of the cases, cathepsin D activity significantly differed between case types (ANOVA F(2,14) = 7.188, p = 0.007), with activity being significantly lower in EOAD compared with healthy ageing (pairwise comparison with Bonferroni p = 0.028), activity did not significantly differ in AD-DS compared with healthy ageing (pairwise comparison with Bonferroni p = 0.070) and no difference in the activity was observed between AD-DS and EOAD (pairwise comparison with Bonferroni p = 1.000) (Additional file 1: Supplementary Fig. 1b). This evidence concerns the gene CTSD and Alzheimer disease.