Increased excretion of thymine and uracil is the biochemical hallmark of DPD deficiency but is also compatible with additional enzyme defects such as dihydropyrimidinase- (dihydropyrimidinuria; MIM #222748) and thymidine phosphorylase deficiency [mitochondrial DNA depletion syndrome 1 (MNGIE type); MIM #603041]. This evidence concerns the gene DPYS and dihydropyrimidine dehydrogenase deficiency.