The m.3685T>C (p.Y127H) variant in MT-ND1 has been reported in one individual with primary mitochondrial disease to date, in a girl with Leigh syndrome (PMID: 35217561). She had developmental delay and regression, generalized tonic clonic seizures, and constipation. Brain MRI showed bilateral asymmetric basal ganglia signal hyperintensities and MRS showed elevated lactate and glucose in the basal ganglia. Labs showed elevated lactate, alanine, and GDF15. Complex I activity was slightly reduced in skin fibroblasts. The variant was present at 62.5% heteroplasmy in blood and was absent in her healthy mother’s blood (PM6_supporting; PMID: 35217561). This variant is absent in the MITOMAP GenBank dataset, gnomAD v3.1.2, and the Helix dataset (PM2_supporting). The computational predictor APOGEE gives a consensus rating of pathogenic with a score of 0.750 (Min=0, Max=1), which predicts a damaging effect on gene function (PP3). 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 October 28, 2024. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PM6_supporting, PM2_supporting, PP3.