Mitochondrial genome sequencing: The entire mtDNA genome was amplified as a single linear amplicon using a high-fidelity long-range PCR (LR-PCR) system (PlatinumTM SuperFiTM Master Mix, Thermo Fisher Scientific), followed by next-generation sequencing (NGS) with an average sequencing depth of 20,000x. Each sample was tested by two independent sets of LR-PCR primers. Comprehensive mtDNA analyses were performed to detect mtDNA single-nucleotide variations (SNVs) and large-scale single and multiple mtDNA deletions by using an in-house custom-built bioinformatics pipeline at the Children's Hospital of Philadelphia Division of Genomic Diagnostic (DGD) CLIA laboratory within the Department of Pathology. The NGS results from two sets of LR-PCR primers were compared. Any low-level heteroplasmic variants that only appear in one primer set would be considered sequencing artifacts and removed. The ultra-deep sequencing coverage can accurately detect SNVs with heteroplasmy levels as low as or close to 1%.