The m.5888delT variant in MT-TY has been reported in one individual with primary mitochondrial disease to date (PMID: 11756614). This woman had bilateral ptosis, chronic progressive external ophthalmoplegia (CPEO), myopathy, and exercise intolerance onset in her 30s. Muscle biopsy showed ragged red fibers, COX-negative fibers, paracrystalline inclusions, and decreased activities of complexes I and IV. The variant was present at 78% in muscle and was undetectable in blood. The variant was also undetectable in blood from her mother, three siblings, and three children (although this cannot be considered a de novo occurrence as the variant was also undetectable in the proband’s blood; PMID 11756614). This variant is absent in the GenBank dataset, Helix dataset, and gnomAD v3.1.2 (PM2_supporting). The computational predictor MitoTIP suggests this variant is pathogenic (53.2 percentile) and HmtVAR predicts it to be pathogenic score of 0.65 (PP3). Single fiber testing showed higher levels of the variant in COX-negative fibers (79.4%) than in COX positive fibers (19.8%), p<0.0001 (PS3_supporting, PMID: 11756614). 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 June 24, 2024. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PM2_supporting, PP3, PS3_supporting.