Guanosine is a nucleoside that consists of guanine, an organic base belonging to the purine family, and the sugar ribose. Guanine, which may be responsible for the iridescence of the scales of some fish species, was found to be a component of bird guano in the first half of the nineteenth century and its structure was suggested as early as 1875. Similar to other nucleosides, one to three phosphoric acid groups may be added to guanosine to produce three nucleotides, including guanosine monophosphate (GMP), guanosine diphosphate (GDP), and guanosine triphosphate (GTP), which is the source of guanosine found in RNA. These nucleotides, as well as those formed from guanine and deoxyribose rather than ribose, carry out a number of significant functions in cellular metabolic processes.