POLG and mitochondrial neurogastrointestinal encephalomyopathy: Interestingly, mtDNA somatic point mutations at the control region have been found in the skeletal muscle of patients with MDDS caused by mutations in POLG and TWNK [62], but to a lesser extent than those found in MNGIE patients caused by mutations in TYMP [63] (see below), which underlines the fact that the generation of somatic point mutations in mtDNA is another molecular feature of MDDS, although less prominent (and likely less functionally relevant) than depletion or multiple deletions.