APOE and PSEN1 mRNA levels are downregulated in blood samples from AD patients, SNCA expression is upregulated in patients with PD, PICALM expression is upregulated in patients with VaD/mixed dementia, and ABCB7 transcript levels are downregulated in subjects with AD and in VaD/mixed dementia. The gene discussed is PSEN1; the disease is Alzheimer disease.