(A) Ratios of copy numbers of human mtDNA and nuclear DNA were determined by qPCR of mtDNA and nuclear DNA marker genes (Mean±SD, triplicated assays). Numbers show fold enrichment compared to total DNA. (B) CpG methylation of mtDNA-encoded genes (box plot; boxes and whiskers indicate quartiles and minimum/maximum values, horizontal lines in boxes indicate median). Numbers at the top indicate p-values of 2-tailed t-test against unmethylated lambda DNA. (C, D) Cytosine methylation of mtDNA (C) and unmethylated lambda DNA (D). Percentage of cytosine methylation in the CpG and non-CpG contexts is shown with red and blue dots, respectively. Deep sequencing read coverage at cytosines is shown with green dots. In panel (C), locations of mtDNA-encoded genes and D-loop are indicated at the top, where positions of tRNA genes are shown with vertical bars without gene names.