TYMP and mitochondrial neurogastrointestinal encephalomyopathy: A subtype of the latter is MDS; a group of mainly autosomal recessive disorders caused by defects in nuclear genes involved in mtDNA replication (e.g., POLG and PEO1 causing hepatocerebal MDS), or genes crucial for maintenance of mtDNA including TK2 (responsible for myopathic MDS), RRM2B (encephalomypathic MDS) and thymidine phosphorylase (TYMP) gene mutations associated with MNGIE (El-Hattab and Scaglia, 2013).