Pyruvate dehydrogenase (PDHB), succinyl-CoA ligase (SUCLG1, SUCLG2, SUCLA2), succinate dehydrogenase (SDHC), and malate Dehydrogenase (MDH1) showed consistent decrease in peripheral blood cells of MCI and AD in both GSE63060 and GSE63061 cohorts, which may be good peripheral blood biomarkers for early diagnosis of AD. The gene discussed is SUCLG1; the disease is Alzheimer disease.