Dihydrouracil belongs to the class of organic compounds known as pyrimidones. Pyrimidones are compounds that contain a pyrimidine ring, which bears a ketone. Pyrimidine is a 6-membered ring consisting of four carbon atoms and two nitrogen centers at the 1- and 3- ring positions. Dihydrouracil is an intermediate breakdown product of uracil. Dihydrouracil exists in all living organisms, ranging from bacteria to plants to humans. Within humans, dihydrouracil participates in a number of enzymatic reactions. In particular, dihydrouracil can be biosynthesized from uracil; which is mediated by the enzyme dihydropyrimidine dehydrogenase [NADP(+)]. The breakdown of uracil is a multistep reaction that leads to the production of beta-alanine. The reaction process begins with the enzyme known as dihydropyrimidine dehydrogenase (DHP), which catalyzes the reduction of uracil into dihydrouracil. Then the enzyme known as dihydropyrimidinase hydrolyzes dihydrouracil into N-carbamyl-beta-alanine. Finally, beta-ureidopropionase catalyzes the conversion of N-carbamyl-beta-alanine into beta-alanine. There is at least one metabolic disorder that is associated with altered levels of dihydrouracil. In particular, dihydropyrimidinase deficiency is an inborn metabolic disorder that leads to highly increased concentrations of dihydrouracil and 5,6-dihydrothymine, and moderately increased concentrations of uracil and thymine in urine. Dihydropyrimidinase deficiency can cause neurological and gastrointestinal problems in some affected individuals (OMIM: 222748). In particular, patients with dihydropyrimidinase deficiency exhibit a number of neurological abnormalities including 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.