A–D , Brain Aβ and amyloid burden in PDAPP mice expressing human apolipoprotein E alleles at 18 months of age. Significantly more Aβ is deposited in the hippocampus ( A , ***p < 0.001, ANOVA, Tukey–Kramer post test) or cortex ( B , ***p < 0.001, **p < 0.01, *p < 0.05, t test) of PDAPP/TRE4 mice at 18 months of age. Significantly more amyloid (thioflavine S material) is present in the hippocampus of PDAPP/TRE4 mice at 18 months of age ( C , **p < 0.01, ANOVA, Tukey–Kramer post test). D , Representative photomicrographs of Aβ immunoreactivity and amyloid (thioflavine S staining) in PDAPP mice expressing various human apoE alleles at 18 months of age.