The m.8043_8044del frameshift variant in MT-CO2 has been reported in one individual with primary mitochondrial disease to date, in an infant with severe lactic acidosis who died at 12 days old (PMID: 11471180). Complex IV deficiency was noted in skeletal and cardiac muscle. The variant was present at 20% heteroplasmy in skeletal muscle, 17% in cardiac muscle, 14% in lung tissue, 14% in liver, and 11.5% in skin. The variant was not assessed in proband’s blood and was undetectable in mother’s blood. Of note, nuclear genetic etiologies were not evaluated in this case. This variant causes a premature stop in the MT-CO2 gene resulting in truncation of 33% of the protein (PVS1_strong). This variant is absent in in the MITOMAP GenBank dataset, gnomAD v3.1.2, and the Helix dataset (PM2_supporting). There are no in silico predictors for this type of variant in mitochondrial DNA. There are no cybrids, single fiber studies, or other functional assays reported on this variant. This variant meets criteria to be classified as uncertain significance for primary mitochondrial disease. After extensive discussion, this Expert Panel agreed with this classification given low heteroplasmy levels in the only reported case with a severe phenotype, limited ability of technology used at the time to accurately report heteroplasmy, and because other genetic etiologies were not excluded in the only case reported. This classification was approved by the NICHD/NINDS U24 ClinGen Mitochondrial Disease Variant Curation Expert Panel on January 22, 2024. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PVS1_strong, PM2_supporting.