Single nucleotide mutations in the mitochondrial genome are linked to aging in humans, primates, and rodents and cause neuromuscular diseases in humans. Load of mitochondrial variants in healthy tissues, however, is little known. Employing an unbiased detection method with no prior enzymatic amplification, we observed that the mitochondrial genome of embryonic, adult, and aged mouse brain from two different strains contains a diversity of single nucleotide variants with no age-related increase in abundance. We also observed de novo variants in single oocytes and adult liver arising at 5x10