Pyrimidine nucleotides play a critical role in cellular metabolism serving as activated precursors of RNA and DNA, CDP diacylglycerol phosphoglyceride for the assembly of cell membranes and UDP-sugars for protein glycosylation and glycogen synthesis. In addition, uridine nucleotides act via extracellular receptors to regulate a variety of physiological processes. There are two routes to the synthesis of pyrimidines; nucleotides can be recycled by the salvage pathways or synthesized de novo from small metabolites. Most cells have several specialized passive and active transporters that allow the reutilization of preformed pyrimidine nucleosides and bases.