The m.3242G>A variant in MT-TL1 has been reported in at least six unrelated individuals with primary mitochondrial disease (PMIDs: 15870203, 19460299, 10214753, 21364701, 14576046, 22781753, 24667782, 31965079, 37038312; PS4_moderate). Clinical features in affected individuals include Leigh syndrome spectrum disorder, encephalomyopathy, myopathy, hypertrophic and dilated cardiomyopathy, and renal insufficiency. The variant occurred de novo in two of these individuals (PMID: 22781753; PM6_supporting). There are no families reported where the variant segregated with clinical manifestations. This variant is absent in the GenBank dataset, Helix dataset, and gnomAD v3.1.2 (PM2_supporting). In silico predictors are conflicting as the computational predictor MitoTIP suggests this variant is likely benign (18.5 percentile) but HmtVAR predicts it to be possibly pathogenic with a score of 0.6. A cybrid study (PMID: 19460299) from cells derived from the index case (PMID: 15870203) demonstrated reduced activity of complex IV. Additional studies showed this variant eliminated transcriptional termination activity with a modest decrease (30%–50%) of mtRNase P activity (PMID: 20550934), and that this variant was associated with decreased single-turnover methyltransferase and cleavage activity of human mitochondrial mtRNase P (PS3_moderate). In summary, this variant meets criteria to be classified as likely pathogenic 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 11, 2024. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PS4_moderate, PS3_moderate, PM2_supporting, PM6_supporting.