Guanine (2-amino-6-oxypurine) is one of the four main nucleobases found in nucleic acids (e.g., DNA and RNA). Guanine is a purine derivative and in Watson-Crick base pairing forms hydrogen bonds in the plane of the fused rings with cytosine. Guanine "stacks" vertically with the other bases via aromatic interactions. Guanine exists as tautomers (see Keto-enol tautomerism), as depicted graphically on the right. The nucleoside is called guanosine.

Guanine is also the name of a white amorphous substance found in the scales of certain fishes, the guano of sea-birds, and the liver and pancreas of mammals.

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