Dihydrouracil is an intermediate breakdown product of uracil. Dihydropyrimidine dehydrogenase (DHP) catalyzes the reduction of uracil into 5,6-dihydrouracil then dihydropyrimidinase hydrolyzes 5,6-dihydrouracil into N-carbamyl-beta-alanine. Finally, beta-ureidopropionase catalyzes the conversion of N-carbamyl-beta-alanine into beta-alanine. When present at abnormally high levels, dihydrouracil can be toxic although the mechanism of toxicity is not clear. In particular, patients with dihydropyrimidinase deficiency exhibit highly increased concentrations of 5,6-dihydrouracil and 5,6-dihydrothymine, and moderately increased concentrations of uracil and thymine can be detected in urine. Dihydropyrimidinase deficiency is a disorder that can cause neurological and gastrointestinal problems in some affected individuals (OMIM: 222748). The neurological abnormalities that occur most often in people with dihydropyrimidinase deficiency are intellectual disability, seizures, weak muscle tone (hypotonia), an abnormally small head size (microcephaly), and autistic behaviours that affect communication and social interaction. Gastrointestinal problems that occur in dihydropyrimidinase deficiency include backflow of acidic stomach contents into the esophagus (gastroesophageal reflux) and recurrent episodes of vomiting. The direct measurement of the activity of DHP in patients has been hampered by the fact that the enzyme is expressed almost exclusively in liver tissue.