The m.4450G>A variant in MT-TM has been reported in two unrelated individuals with primary mitochondrial disease (PS4_supporting). The first individual reported was a girl with childhood onset myopathy (fatigue, proximal muscle weakness, exercise intolerance) and developmental delay. Muscle biopsy revealed COX-negative fibers and a combined respiratory chain enzyme deficiency. The variant was present at 67% in muscle, 10% in fibroblasts, and was undetectable in blood and buccal samples (PMID: 25468263). The variant was undetectable in her healthy mother’s blood, muscle, and urinary sediment (PS2_moderate). The second individual reported was a girl with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) and the variant was present at varying levels (20-59%) in several tissues tested. Fibroblasts in this individual showed reduced oxygen consumption and reduced activities of respiratory chain complexes I and IV (PMID: 30739820). There are no other large families reported in the medical literature to consider for evidence of segregation. The computational predictor MitoTIP suggests this variant is pathogenic (83.7 percentile) and HmtVAR predicts it to be pathogenic score of 1 (PP3). This variant is absent in the GenBank dataset, Helix dataset, and gnomAD v3.1.2 (PM2_supporting). Single fiber testing showed high heteroplasmy levels in ragged blue fibers (94% ± 5.4%) and variant was not detected in normal appearing fibers (PMID: 25468263, PS3_supporting). In summary, this variant meets criteria to be classified as likely pathogenic 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 February 27, 2023. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PS4_supporting, PS2_moderate, PM2_supporting, PP3, PS3_supporting.