Nucleotides and their derivatives are used for short-term energy storage (ATP, GTP), for intra- and extra-cellular signaling (cAMP; adenosine), as enzyme cofactors (NAD, FAD), and for the synthesis of DNA and RNA. All of the nucleotides can be synthesized de novo. Additional metabolic pathways allow the interconversion of nucleotides, the salvage and reutilization of nucleotides released by degradation of DNA and RNA, and the catabolism of excess nucleotides. These pathways are regulated to control the total size of the intracellular nucleotide pool, to balance the relative amounts of individual nucleotides, and to couple the synthesis of deoxyribonucleotides to the onset of DNA replication (S phase of the cell cycle). The catabolism of purines via inosine monophosphate (IMP) to urate is the major route by which excess nitrogen is excreted from the body in chickens and other birds.