Uridine 5'-diphosphate, also known as 5'-UDP, UDP or uridine diphosphoric acid, belongs to the class of organic compounds known as pyrimidine ribonucleoside diphosphates. These are pyrimidine ribonucleotides with diphosphate group linked to the ribose moiety. UDP is also classified as a nucleotide diphosphate. It is an ester of pyrophosphoric acid with the nucleoside uridine. UDP consists of a pyrophosphate group, a pentose sugar ribose, and the nucleobase uracil. UDP exists in all living species, ranging from bacteria to plants to humans. In mammals UDP is an important factor in glycogenesis or the formation of glycogen in the liver. Before glucose can be stored as glycogen in the liver and muscles, the enzyme UDP-glucose pyrophosphorylase forms a UDP-glucose unit by combining glucose 1-phosphate with uridine triphosphate, cleaving a pyrophosphate ion in the process. Then, the enzyme glycogen synthase combines UDP-glucose units to form a glycogen chain. UDP is also an important extracellular pyrimidine signaling molecule that mediates diverse biological effects via P1 and P2 purinergic receptors, such as the uptake of thymidine and proliferation of gliomas. UDP plays a key role in the function of Uridine 5'-diphospho-glucuronosyltransferases (UDP-glucuronosyltransferases, UGTs) which catalyze the transfer of the glucuronic acid component of UDP-glucuronic acid to a small hydrophobic molecule. UDP-Glucuronosyltransferases are responsible for the process of glucuronidation, a major part of phase II metabolism. The reaction catalyzed by UGT enzymes involves the addition of a glucuronic acid moiety to xenobiotics and is the most important pathway for the human body's elimination of the most frequently prescribed drugs. It is also the major pathway for foreign chemical (dietary, environmental, pharmaceutical) removal for most drugs, dietary substances, toxins and endogenous substances. UGT is present in humans, other animals, plants, and bacteria. Famously, UGT enzymes are not present in the genus Felis (PMID: 10862526) and this accounts for a number of unusual toxicities in the cat family.