At least 12 nuclear genes have been found to be involved in mtDNA maintenance, including POLG, POLG2, PEO1, SLC25A4, TYMP, DGUOK, TK2, SUCLA2, SUCLG1, MPV17, OPA1 and RRM2B genes.1 MDS are caused by recessive defects in proteins directly involved in mtDNA replication or in proteins that affect the availability of deoxyribonucleoside triphosphates for mtDNA synthesis. The gene discussed is TWNK; the disease is myelodysplastic syndrome.