Cordycepin, also known as 3'-Deoxyadenosine, is a purine nucleoside antimetabolite and antibiotic isolated from the fungus Cordyceps militaris with potential antineoplastic activity. Cordycepin is an adenosine analogue, which is readily phosphorylated to its mono-, di-, and triphosphate intracellularly. Triphosphate cordycepin can be incorporated into RNA, and inhibits transcription elongation and RNA synthesis due to the absence of a hydroxyl moiety at the 3' position. Because it can be converted to an inactive metabolite by adenosine deaminase, this agent must be administered with an adenosine deaminase inhibitor in order to be effective. Cordycepin has displayed cytotoxicity against some leukemic cell lines in vitro.