The m.7671T>A (p.M29K) variant in MT-CO2 has been reported in two unrelated individuals with primary mitochondrial disease to date. The first individual was a 14 year old male with proximal myopathy, lactic acidosis, and a five year history of muscle weakness and fatigue. Skeletal muscle biopsies revealed severe COX deficiency, increased SDH staining, and absence of ragged red fibers. Variant heteroplasmy was 90% in muscle and 4.5% - 6% in blood. The second individual was a 48 year old male of European ancestry with mitochondrial myopathy characterized by progressive weakness. Serum creatine kinase levels were elevated and muscle biopsy revealed scattered COX-negative fibers with increased SDH staining. This variant was observed at 52% heteroplasmy, presumably in the muscle sample (PMID: 32800583). Haplogroup information was not reported for both cases precluding consideration for PS4. This variant occurred de novo in one individual (absent in blood from mother; PM6_supporting, PMID: 10486321). Single fiber testing showed higher levels of the variant in COX negative fibers (81%, n=13) than in COX positive fibers (45%, n=4), p<0.01 (PS3_supporting, PMID: 10486321). The computational predictors APOGEE1 and APOGEE2 give a consensus rating of pathogenic with raw scores of 0.90 and 0.942, respectively (Min=0, Max=1), which predict a damaging effect on gene function (PP3). This variant is absent in the GenBank dataset, Helix dataset, and gnomAD v3.1.2 (PM2_supporting). 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 28, 2023. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PM6_supporting, PS3_supporting, PM2_supporting, PP3.