TYMP and mitochondrial neurogastrointestinal encephalomyopathy: We can speculate that massive plasma accumulation of thymidine in MNGIE patients results in consistent mtDNA depletion in muscle whereas moderate levels of thymidine observed in late-onset TYMP-patients prime the age-dependent accumulation of mtDNA deletions but are not sufficient to significantly affect mtDNA content and polymerase fidelity.