TWNK and hereditary disease: Somatic mosaicism is the phenomenon of a single variant occurring in 2 or more populations of soma cells in 1 individual and has been increasingly recognized as an important pathogenic mechanism in genetic disease.5 Probable germline mosaicism of dominant pathogenic TWNK and SLC25A4 variants causing adult-onset PEO with multiple mtDNA deletions have been previously described.6,7 By contrast, we present evidence of somatic mosaicism for a pathogenic TWNK variant associated with this disorder.