A nucleotide is a phosphorylated nucleoside. Ribonucleosides may be phosphorylated via the oxygen atoms found at C2, C3, or C5 of the ribose ring. Ribonucleotides have one, two, or three phosphate groups attached to the ribose sugar. In some ribonucleosides, a phosphorus group is bound to two oxygen atoms of the ribose ring, yielding a cyclic nucleotide. Some important examples of these include cyclic 3′,5′ adenosine monophosphate (cAMP), an important cellular signaling molecule, and the 2′,3′ cyclic nucleotides which may be formed during the nonenzymatic hydrolysis of polyribonucleotides. Ribonucleotides are incorporated into nucleic acids as their triphosphates, with the liberation of pyrophosphate during polymerization. An extremely important energy-carrying ribonucleotide is adenosine triphosphate ( ATP).