Once inside the cell, Cordycepin gets converted into 5′ mono-, di- and tri-phosphates that inhibit the activity of enzymes like ribose-phosphate pyrophosphokinase and 5-phosphoribosyl-1-pyrophosphate amidotransferase which are used in de novo biosynthesis of purines (Fig. 2) (Klenow 1963; Overgaard 1964; Rottman and Guarino 1964).