APOE and Alzheimer disease: Meanwhile, our study of the mtDNA copy number in the peripheral leukocytes found 2.37 ± 0.19, 2.31 ± 0.24, 2.22 ± 0.17 in the three individual APOE4 groups of the AD cohort, and 2.47 ± 0.29, 2.44 ± 0.26, 2.36 ± 0.20 in the non-AD cohort.