The m.8618dupT (p.Thr33Hisfs*32) variant in MT-ATP6 has been reported in two unrelated individuals with primary mitochondrial disease (PS4_supporting; PMIDs: 32042910, 19124644). Clinical features in these individuals include developmental delay, learning difficulties, ataxia, spastic paraparesis, headache, cerebellar atrophy, white matter abnormalities, hearing loss, kidney failure, diabetes, cataracts, optic atrophy, retinal dystrophy, and short stature. The heteroplasmy levels in affected individuals ranged from 20% to 85%. This variant occurred de novo in one individual (absent in blood from mother, sister, and maternal aunt; PM6_supporting, PMID: 19124644). This variant segregated with disease in the other family (proband with heteroplasmy levels ranging from 20-65%; unaffected individuals including mother, sister, and maternal aunt with lower heteroplasmy levels ranging from 0-24%; PP1; PMID: 32042910). This frameshift variant results in a significant (>10%) truncation of the MT-ATP6 protein (PVS1_strong). This variant is absent in the GenBank dataset, Helix dataset, and gnomAD v3.1.2 (PM2_supporting). There are no in silico predictors for this type of variant in mitochondrial DNA. 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 ClinGen Mitochondrial Disease Variant Curation Expert Panel on August 12, 2024. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PS4_supporting, PM6_supporting, PP1, PM2_supporting, PVS1_strong.