TK2 and myelodysplastic syndrome: Mutations in two other genes encoding mitochondrial thymidine kinase 2: TK2 [67] and cytosolic P53-dependent 2B subunit of ribonucleotide reductase, RRM2B [68], are both associated with severe MDS in skeletal muscle, while defects of the ATP-dependent succinyl-CoA ligase, SUCLA2, cause multisystem, predominantly encephalopathic syndromes, which combine MDS with the presence of methylmalonic acid in body fluids [69,70,71].