The m.13051G>A (p.G239S) variant in MT-ND5 has been reported in thirteen individuals with primary mitochondrial disease from four kindreds (PS4_moderate; PMIDs: 12736867, 27164671). Age of onset varied from age 18 months to the 40s, and clinical features were consistent with Leber Hereditary Optic Neuropathy (LHON) and Leigh syndrome spectrum disorders, in addition to hearing loss, optic atrophy, retinal dystrophy, ophthalmoplegia, and seizures. Several individuals had elevated lactate levels. The variant was present at or near homoplasmy in all reported individuals, thus precluding consideration for segregation evidence. There are no reports of confirmed de novo occurrences to our knowledge. This variant is absent in the GenBank dataset and gnomAD v3.1.2, however there were three heteroplasmic occurrences in the Helix dataset (PM2_supporting). The computational predictor APOGEE gives a consensus rating of pathogenic with a score of 0.71 (Min=0, Max=1), which predicts a damaging effect on gene function (PP3). Comprehensive analysis in patient cells showed increased reactive oxygen species and increased mitophagy (PS3_supporting). In summary, this variant meets criteria to be classified as uncertain significance for primary mitochondrial disease inherited in a mitochondrial manner, although this Expert Panel notes this is a very compelling variant of uncertain significance. This classification was approved by the NICHD/NINDS U24 ClinGen Mitochondrial Disease Variant Curation Expert Panel on May 23, 2023. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PS4_moderate, PM2_supporting, PP3, PS3_supporting.