The m.10010T>C variant in MT-TG has been reported in six unrelated individuals with primary mitochondrial disease. Features in affected individuals include Leigh syndrome (in two individuals), lactic acidosis, hyperCKemia, developmental delay, exercise intolerance, seizures, ataxia, sensorineural hearing loss, and optic atrophy. Muscle biopsies revealed ragged red fibers, COX-deficient fibers, and reduced activities of complexes I, III, and IV. The heteroplasmy levels of the variant in affected individuals ranged from 5.4% in blood to 92% in muscle. In cases that had more than one tissue tested, the heteroplasmy level was consistently and significantly higher in muscle (PS4_moderate; PMIDs: 26469001, 11971101, 16120360, 23847141, 9199564). There are no reported de novo occurrences of this variant to our knowledge. There are no large families reported in the medical literature to consider for evidence of segregation. This variant is absent in the GenBank dataset, Helix dataset, and gnomAD v3.1.2 (PM2_supporting). Single fiber testing in two separate individuals showed higher levels of the variant in COX-negative fibers (97.3% and 91.7%) than in COX-positive fibers (87.2% and 58%) (PS3_supporting, PMIDs: 9199564, 11971101). The computational predictor MitoTIP suggests this variant is pathogenic (82.2 percentile) and HmtVAR predicts it to be pathogenic score of 0.55 (PP3). In summary, this variant meets criteria to be classified as uncertain significance for primary mitochondrial disease inherited in a mitochondrial manner. This classification was approved by the NICHD/NINDS U24 ClinGen Mitochondrial Disease Variant Curation Expert Panel on November 14, 2022. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PS4_moderate, PM2_supporting, PS3_supporting, PP3.