Adenosine is an organic compound that occurs widely in nature in the form of diverse derivatives. The molecule consists of an adenine attached to a ribose via a β-N9-glycosidic bond. Adenosine is one of four nucleoside building blocks to RNA, which is essential for all life. Its derivatives include the energy carriers adenosine mono-, di-, and triphosphate. Cyclic adenosine monophosphate (cAMP) is pervasive in signal transduction. Adenosyl (Ad) is the radical formed by removal of the 5′-hydroxy (OH) group. Ad is found in vitamin B12 and the radical SAM enzymes. Adenosine is also used as a medication.