The m.6721T>C (p.M273T) variant in MT-CO1 has been reported in one individual to date, in a 58-year-old woman with acquired idiopathic sideroblastic anemia (PMID: 9389715). The variant was present in bone marrow from the proband and absent in her mother’s blood (PM6_supporting). There have not been additional affected individuals reported to date. This variant is absent in the GenBank dataset, Helix dataset, and gnomAD v3.1.2 (PM2_supporting). The computational predictor APOGEE gives a consensus rating of pathogenic with a score of 0.72 (Min=0, Max=1; APOGEE2 score is 0.834), which predicts a damaging effect on gene function (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 March 11, 2024. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PP3, PM2_supporting, PM6_supporting.