The m.11778G>A (p.R340H) variant in MT-ND4 has been reported in >16 unrelated individuals with primary mitochondrial disease. 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, affected individuals have been reported with features including LHON, Leigh syndrome, cerebellar ataxia, migraines, regression, leukoencephalopathy, myoclonus, psychiatric illness, Parkinsonism, axonal neuropathy, multiple sclerosis, and ophthalmoplegia; and this variant has been seen in affected individuals in the homoplasmic and heteroplasmic states (PS4; PMIDs: 3201231, 2575667, 2566021, 2390098, 1635296, 17724295, 18848389, 25917160, 18216301, 17254817, 8902729, 27119776). 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: 27119776, 2390098). There are no confirmed de novo occurrences of this variant to our knowledge. 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 0.97 (Min=0, Max=1), which predicts a damaging effect on gene function (PP3). Cybrid studies supported the functional impact of this variant (PS3_supporting; PMIDs: 10976107, 7763260). This variant meets criteria to be classified as likely pathogenic however this Expert Panel elected to modify the classification to pathogenic given the overwhelming evidence of pathogenicity. Furthermore, the mitochondrial DNA variant specifications are known to not be optimized for pathogenic variants that tend to be homoplasmic in nature and/or have reduced penetrance, such as the common variants associated with LHON. This classification was approved by the NICHD/NINDS U24 Mitochondrial Disease Variant Curation Expert Panel on May 24, 2022. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PS3_supporting, PS4, PP1_moderate, PP3.