The m.5816A>G variant in MT-TC has been reported in two unrelated families with primary mitochondrial disease (PS4_supporting; PMIDs: 17724295, 37771542). The first reported family had four affected individuals with variable clinical features including dystonia, seizures, tremor, fatigue, ptosis, pigmentary retinopathy, and optic atrophy. Muscle biopsies in these family members showed mosaic COX deficiency, normal respiratory chain enzyme activities, and decreased steady state levels of mt-tRNA Cys. The variant was homoplasmic in muscle and blood in these individuals (PMID: 17724295). The second family reported had one affected individual, a girl, with dystonia, episodic vomiting, motor regression, tremor, and seizures. Skin fibroblast testing showed reduced basal resting oxygen consumption rate (OCR), ATP production, proton leak, maximal respiration, and reserve capacity OCR. The variant was present in blood and urine but heteroplasmy levels were not reported (PMID: 37771542). The variant was present in additional family members of this second family however phenotypes and heteroplasmy levels were not reported. This variant is absent in the GenBank dataset, Helix dataset, and gnomAD v3.1.2 (PM2_supporting). There are no cybrids, single fiber studies, or other functional assays reported on this variant. The computational predictor MitoTIP suggests this variant is pathogenic (59.9 percentile) and HmtVAR predicts it to be pathogenic score of 0.65 (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 April 23, 2024. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PS4_supporting, PM2_supporting, PP3.