TYMP and mitochondrial neurogastrointestinal encephalomyopathy: Nucleotide pools are finely balanced and instances of elevated mitochondrial dTTP have been shown to cause reduced mtDNA copy number in patients with MNGIE, due to pathogenic variants in TYMP [37] and high equimolar concentrations of supplemented deoxynucleosides have been shown to cause toxicity in vitro [27].