The m.12258C>T variant in MT-TS2 has been reported in one individual with primary mitochondrial disease to date however clinical details were not provided (PMID: 31965079). There are no reports of large families with this variant segregating with disease. There are no reported de novo occurrences of this variant to our knowledge. Another variant at this position was classified by this Expert Panel as likely pathogenic (m.12258C>A; PM5_supporting). There is one heteroplasmic occurrence of this variant in the Helix dataset and this variant is absent in the MITOMAP GenBank dataset and in gnomAD v3.1.2 (PM2_supporting). In silico predictors are conflicting as the computational predictor MitoTIP suggests this variant is pathogenic (81.5 percentile) but HmtVAR predicts it to be neutral with a score of 0.05. There are no cybrids, single fiber studies, or other functional assays reported on this variant. 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 January 7, 2025. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PM2_supporting, PM5_supporting.