The m.5549G>A variant in MT-TW has been reported in two individuals with primary mitochondrial disease to date (PMIDs: 7695240, 34276539; PS4_supporting). The first reported case was a man with onset at age 40 years of personality and behavioral changes followed by progressive cognitive decline, hearing loss, ataxia, chorea, dysarthria, and axonal neuropathy, and he died at age 53 years (PMID: 7695240). Brain imaging showed atrophy of the cerebrum, cerebellum, and brainstem. Muscle biopsy showed COX-negative and ragged red fibers. The variant was present at 40% heteroplasmy in blood, 83-86% in skeletal muscle, and 51-93% in organs on autopsy (liver, optic nerve, lung, heart, kidney, cerebral cortex, cerebellum). There was no mention of testing in family members. The second reported case was a 26-year-old man with onset in adolescence of epilepsy, ataxia, dystonia, impaired cognition, and hearing loss (PMID: 34276539). Brain imaging showed leukoencephalopathy and cerebral atrophy in addition to calcifications of the bilateral basal ganglia, thalamus, and cerebellar dentate nuclei. Muscle biopsy showed ragged blue and COX-deficient fibers. The variant was present at 5.4% heteroplasmy in blood and 61.5% in muscle. The variant was absent in his mother’s blood (PMID: 34276539; PM6_supporting). This variant is absent in the MITOMAP GenBank dataset, gnomAD v3.1.2, and the Helix dataset (PM2_supporting). The computational predictor MitoTIP suggests this variant is pathogenic (88.3 percentile) and HmtVAR predicts it to be pathogenic with a score of 0.35 (PP3). There are no cybrids, single fiber studies, or other functional assays reported on this variant. 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): PS4_supporting, PM6_supporting, PM2_supporting, PP3.