The m.3697G>A (p.G131S) variant in MT-ND1 has been reported in at least 14 individuals with primary mitochondrial disease from 8 families. Affected individuals had onset ranging from the first week of life to adulthood; and with features variably consistent with Leigh syndrome, MELAS, and LHON (PS4_moderate; PMIDs: 31996177, 30623604, 28429146, 24830958, 18977334, 17562939, 15466014). There are no reports of de novo occurrence of this variant. 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: 24830958, 17562939, 16969869, 15466014). This variant is absent in the GenBank dataset, Helix dataset, and gnomAD v3.1.2 (PM2_supporting). Functional studies supported the deleterious impact of this variant (PS3_supporting; PMID: 15466014). 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 March 22, 2022. Mitochondrial DNA-specific ACMG/AMP criteria applied: PS3_supporting, PS4_moderate, PM2_supporting, PP1_moderate.