The m.3274A>G in MT-TL1 has been reported in one individual to date, in a man with a progressive neuropsychiatric disorder and multisystem organ involvement. At 12 years old, he had a decline in school performance. By his late 20s, he experienced acute psychotic symptoms and depression. He also had bilateral hearing loss, gait instability, and bilateral dysdiadokinesis. By his early 30s, he had cataracts, retinal degeneration, muscle weakness, ataxic gait, dysarthria, intention tremor, and dysmetria. Brain MRIs showed progressive cerebral and cerebellar atrophy and T2 hyperintense lesions in the basal ganglia. He had elevated blood and cerebrospinal fluid lactate. Muscle biopsy showed increased central nuclei and ragged red fibers, with reduced complex I activity (53% of lowest norm). The variant was present at 25% in muscle and undetectable in blood (PMID: 11723298). The variant was absent in blood from three healthy maternal family members, but this cannot be considered evidence of de novo status as the variant was also absent in the proband’s blood. This variant is absent in the GenBank dataset, Helix dataset, and gnomAD v3.1.2 (PM2_supporting). Single fiber testing showed higher levels of the variant in ragged red fibers (55 ± 26.1%; N=22; mutant mtDNA found in every fiber) than in normal fibers (14 ± 25.7%; N=16; mutant mtDNA found in 7/16 fibers), p<0.001 (PS3_supporting, PMID: 11723298). The computational predictor MitoTIP suggests this variant is pathogenic (77.1 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 April 23, 2024. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PS3_supporting, PM2_supporting, PP3.