Based on data obtained from patients referred to the London and Oxford NHS England nationally commissioned service for mitochondrial diseases, RNASEH1 mutations represent the fourth most common cause of mendelian PEO associated with multiple mtDNA deletions in adults (2.7%, 3/109), following mutations in POLG (24.7%, 27/109), RRM2B (16.5%, 18/109), and TWNK (also known as C10orf2, PEO1, or Twinkle, 16.5%, 18/109). The gene discussed is TWNK; the disease is mitochondrial disease.