The m.5545C>T variant in MT-TW has been reported in one individual with primary mitochondrial disease to date, in a boy with hypertrophic cardiomyopathy, hypotonia, lactic acidosis, developmental regression, seizures, slowly progressive chorea, proximal muscle weakness, ataxia, and dysmetria. Brain MRI showed diffuse brain atrophy and bilateral putaminal intensities. Muscle biopsy showed a diffuse reduction in COX activity with normal SDH staining. Respiratory chain enzyme activities in muscle showed reduced complex IV activity (12% of controls). The variant was present at 25% heteroplasmy in muscle, 13% in leukocytes, and 17% in skin fibroblasts (PMID: 18337306). The variant was absent in leukocytes, urine, hair, and buccal sample from his mother (PMID: 18337306; PM6_supporting). There are no other large families reported to consider evidence for segregation. This variant is absent in the GenBank dataset, Helix dataset, and gnomAD v3.1.2 (PM2_supporting). Single fiber testing showed higher levels of the variant in COX-negative fibers (31±7%, n = 11) than in COX-positive fibers (3 ± 4%, n = 13), p<0.01 (PS3_supporting, PMID: 18337306). Cybrid studies and translation assay further support the functional effect of this variant (PMID: 18337306). The computational predictor MitoTIP suggests this variant is pathogenic (53 percentile) and HmtVAR predicts it to be pathogenic score of 0.7 (PP3). 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 April 23, 2024. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PM6_supporting, PM2_supporting, PS3_supporting, PP3.