The m.8993T>C (p.L156P) variant in MT-ATP6 has been reported in >16 individuals with primary mitochondrial disease with onset ranging from the first year of life to adulthood; and who had features variably consistent with Leigh syndrome and neurogenic muscle weakness, ataxia, and retinitis pigmentosa (NARP) (PS4; PMIDs: 8395787, 16532470, 30128709, 29101127, 28003964, 26265210, 22819295, 19046652, 18055910, 16049925, 10604142). Per our literature review and a recently published review, there are no published de novo occurrences of this variant (PMID: 30763462). This variant is located at the same amino acid position as another well-known pathogenic variant, m.8993T>G (p.L156R) (PM5). This variant segregated with disease in multiple affected members in multiple families and several healthy family members had lower to undetectable levels of the variant (PP1_moderate; PMID: 10604142). In silico tools predict this variant to be pathogenic (PP3). Cybrid studies (homoplasmic for this variant) showed reduced ATP production compared to Rho+ control cell lines (PS3_supporting; PMID: 19160410). In summary, this variant meets criteria to be classified as pathogenic for primary mitochondrial disease inherited in a maternal manner. This classification was approved by the NICHD U24 Mitochondrial Disease Variant Curation Expert Panel on February 17, 2021. Mitochondrial DNA-specific ACMG/AMP criteria applied: PS3_supporting, PS4, PM5, PP1_moderate, PP3).