The m.3394T>C (p.Y30H) variant in MT-ND1 has been reported in 157 individuals with phenotypes consistent with primary mitochondrial disease, including Leber Hereditary Optic Neuropathy (LHON) and diabetes. This variant was present in 2.1% of 1,741 individuals in one cohort of LHON, and is thought to increase penetrance and occurrence of optic neuropathy in families when present with the well-known pathogenic variant, m.11778G>A (PMID: 30597069). The m.3394T>C variant is widely considered a secondary "helper" variant, not a variant associated with primary mitochondrial disease. However, of the 157 cases reported to date, the m.3394T>C variant has been reported without an accompanying primary mitochondrial disease-associated variant in 31 LHON families, two individuals with diabetes, and one individual with hypertrophic cardiomyopathy (PS4). Although outside the scope of this curation, this variant also been shown to be adaptive for high-altitude Tibetans where multiple independent origins of m.3394T>C have been documented (PMID: 22517755). The variant is present in 100% in Tibetan-associated haplogroups M9a (349/349) and M9b (4/4) in MITOMAP. In a study of type 2 diabetes mellitus in Han Chinese, this variant was found with significantly higher frequency in affected individuals (23.6%) than in the controls (2.0%; PMID: 18679013). The computational predictor APOGEE gives a consensus rating of pathogenic with a score of 0.76 (Min=0, Max=1; APOGEE2 score is 0.582), which predicts a damaging effect on gene function (PP3). Cybrid studies showed that this variant can either be deleterious or beneficial depending on haplogroup and environmental context. The variant reduces both complex I activity and NADH-linked respiration especially when on non-M9 haplogroups. In Asia, this variant is most commonly associated with the M9 haplogroup, which is rare at low elevations but increases in frequency with elevation to an average of 25% of the Tibetan mtDNAs (odds ratio = 23.7). In high-altitude Tibetan and Indian populations, this variant is enriched on the M9 background in Tibet. When present on the M9 background, this variant is associated with a complex I activity that is equal to or higher than that of the wild type sequence on other haplotype backgrounds (B4c and F1; PMID: 22517755). In a cybrid study of East Asian metabolic syndrome (PMID: 29997041), complex I activity of the cells with this variant had lower complex I activity than cells with the wild type sequence, and that these lower complex I levels correlated with increased risk for metabolic phenotypes. Addition of this variant further reduced the complex I activity on cybrids of B4c, F1, and M9 haplogroup backgrounds (PS3_supporting). 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 September 11, 2023. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PP3, PS3_supporting, PS4.