The m.5556G>A variant in MT-TW has been reported in one individual with mitochondrial disease to date (PMID: 19809478). This was a girl with feeding difficulty and reflux onset shortly after birth and developmental regression beginning at age seven months. She also had constipation, failure to thrive, and died at 13 months due to respiratory insufficiency. She had metabolic acidosis and elevated blood and cerebrospinal fluid lactate. There was reduced activities of complexes I, III, and IV in skeletal muscle and fibroblasts, and reduced levels of fully assembled complexes I, III, IV, and V in fibroblast cell line. Mitochondrial protein synthesis and tRNA tryptophan levels were reduced in patient fibroblasts. The variant was present at 92% heteroplasmy in fibroblasts and 93% in skeletal muscle. The variant was not detectable in mother’s blood, hair, urine, or skeletal muscle (PM6_supporting; PMID: 27450679). This variant is absent in the MITOMAP GenBank dataset, gnomAD v3.1.2, and the Helix dataset (PM2_supporting). In silico prediction tools are conflicting as the computational predictor MitoTIP suggests this variant is tolerated (44.5 percentile) but HmtVAR predicts it to be deleterious (score of 0.85). Cybrids with high heteroplasmy levels (90%, 93%, 96%) were generated and showed a pattern consistent with patient fibroblasts, and tRNA tryptophan levels were reduced in cybrids to 33% (PS3_supporting; PMID: 19809478). In summary, this variant meets criteria to be classified as uncertain significance for primary mitochondrial disease however, after extensive discussion, this Expert Panel elected to modify the classification to likely pathogenic given the high heteroplasmy levels correlating with a severe phenotype with confirmed de novo status of the variant and compelling functional validation. This classification was approved by the NICHD/NINDS U24 ClinGen Mitochondrial Disease Variant Curation Expert Panel on May 28, 2024. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PM6_supporting, PM2_supporting, PS3_supporting.