The m.9185T>C (p.L220P) variant in MT-ATP6 has been reported in more than 16 individuals with features of primary mitochondrial disease with manifestations including Leigh syndrome, MELAS, NARP, neuropathy, ataxia, exercise intolerance, muscle weakness, developmental delay, and seizures (PS4; PMIDs: 28429146, 33717984, 27290639, 24153443, 27783406, 32042921, 31500933, 29116603, 31187502, 22577227, 25548692, 18461509, 17352390, 29228836, 16217706). There are at least two reports of de novo occurrences of this variant (PM6; PMIDs: 33717984, 29228836). This variant segregated with disease in multiple affected members in multiple families and several healthy family members had lower levels of the variant (PP1_moderate; PMIDs: 16217706, 17352390,20546952). This variant is absent in population databases after removing sequences from individuals with mitochondrial disease (three occurrences in GenBank sequences are from individuals with mitochondrial disease; absent in gnomAD v3.1.2 and Helix dataset; PM2_supporting). Studies in cybrids support the functional impact of this variant (PS3_supporting; PMID: 24153443). The computational predictor APOGEE gives a consensus rating of pathogenic with a score of 0.94 (Min=0, Max=1), which predicts a damaging effect on gene function (PP3). 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 U24 Mitochondrial Disease Variant Curation Expert Panel on May 20, 2021. Mitochondrial DNA-specific ACMG/AMP criteria applied: PS4, PM6, PS3_supporting, PP1_moderate, PM2_supporting, PP3.