The m.6742T>C (p.I280T) variant in MT-CO1 has been reported in one individual to date, in a man with sideroblastic anemia (PMID: 9389715). The variant was present at 52% in bone marrow, 59% in peripheral blood, 71% in platelets, and was undetectable in buccal and fibroblasts. As this is the only case reported to date, PS4 could not be applied. There was no mention of family member testing precluding consideration for de novo status or segregation. The computational predictor APOGEE gives a consensus rating of pathogenic with a score of 0.63 (Min=0, Max=1), which predicts a damaging effect on gene function (PP3). This variant is absent in the GenBank dataset and gnomAD v3.1.2, and there is one heteroplasmic occurrence in the Helix dataset (0.001%, haplogroup I; PM2_supporting). 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 December 21, 2023. Mitochondrial DNA-specific ACMG/AMP criteria applied (PMID: 32906214): PM2_supporting, PP3.