Mitochondrial heteroplasmy is elevated in older mouse tissues. The number of heteroplasmic sites is the amount of nucleotide positions at which an alternative allele was observed in a given sample. A) Only seven unique heteroplasmic variants were found across all tissues of the younger cohort. B) Average heteroplasmy is the average frequency of alternative alleles that were observed in a sample. C) Finally, cumulative heteroplasmic burden is the additive frequency of all variants found in one sample. Across the three aforementioned metrics, there was a significant increase in the older mouse tissues (Student's t test, P-values shown above each plot. ***P < 0.0005, ****P < 0.00005). The colors (see key) represent the tissue from which each mtDNA sample was taken from. The number of samples were as follows: 1 year (n = 26) and 2 years (n = 22); brain (n = 4 and 3, respectively), heart (n = 4 and 3, respectively), kidney (n = 4 and 4, respectively), liver (n = 3 and 3, respectively), lungs (n = 4 and 4, respectively), muscle (n = 3 and 1, respectively), and spleen (n = 4 and 4, respectively).