TYMP and mitochondrial neurogastrointestinal encephalomyopathy: In some MNGIE cases there is no or mild clinical involvement of gastrointestinal tract or skeletal muscle, despite the presence of mutations in the TYMP gene leading to marked reduction in TP activity, probably indicating that environmental factors contribute to the severity of the clinical symptoms (Martin et al., 2004; Szigeti et al., 2004b).