The m.3635G>A (p.S110N) variant in MT-ND1 has been reported in >16 individuals with primary mitochondrial disease, and more specifically Leber's hereditary optic neuropathy (LHON). This variant has been observed in affected individuals primarily in the homoplasmic state to date (20/21 cases) (PS4; PMIDs: 25194554, 21074518, 23304069,19527690, 19497304, 11479733, 33417421). There are no large families reported in the medical literature to consider for evidence of segregation. There are no reported de novo occurrences of this variant to our knowledge. This variant is absent in Helix and the GenBank dataset. It is present in gnomAD 3.1 in 1/56428 individuals (0.002%). Although there is one occurrence, the frequency is still low (PM2_supporting). The computational predictor APOGEE gives a consensus rating of pathogenic with a score of 0.75 (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: 25194554, 11479733). 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 Mitochondrial Disease Variant Curation Expert Panel on August 23, 2022. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PM2_supporting, PS3_supporting, PP3, PS4.