The m.3460G>A (p.A52T) variant in MT-ND1 has been reported in affected individuals from more than 50 kindreds (PS4, PMIDs: 1928099, 1674640, 7629530, 1734726, 1550131, 8496715, 8024249, 8556281, 8571959, 12205655, 11906302, 12807863, 12518276, 16738010, 17122117, 18216301, 18562849, 20232220, 21887510, 25338955, 25053773, 28314831, 30053855, 30591017). While this variant is one of the three most common variants associated with Leber Hereditary Optic Neuropathy (LHON; PMID: 20301353), affected individuals can also have other features. Indeed, several individuals have been reported with multiple sclerosis. Other features have been variably seen in affected individuals including sensorineural hearing loss, auditory neuropathy, epilepsy, migraines, Parkinsonism, dystonia, Leigh syndrome, spinal cord lesions, myoclonus, myopathy, hypertension, high triglycerides, diabetes, and cardiac involvement. Testing excluding separate etiologies for these features has been limited. Heteroplasmy levels in affected individuals ranged from 58% to homoplasmy. Age of onset varied from four years old to 75 years old. This variant segregated with disease in multiple affected members in multiple families and several healthy family members had lower to undetectable levels of the variant (PP1_moderate; PMIDs: 1734726, 8571959, 11906302). There is one reported de novo occurrence to our knowledge (PM6_supporting; PMID: 12518276). This variant is present in the healthy population, which is to be expected given the known reduced penetrance of this variant. The computational predictor APOGEE gives a consensus rating of pathogenic with a score of 0.86 (Min=0, Max=1), which predicts a damaging effect on gene function (PP3). Extensive cybrid studies supported the functional impact of this variant, as do E. coli and mouse studies (PS3_moderate PMIDs: 35383288, 22079202, 15720387, 15883259, 15342361, 10976107). In summary, this variant meets criteria to be classified as 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 April 25, 2023. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PS4, PP1_moderate, PM6_supporting, PP3, PS3_moderate.