The information in Figure 14-6, which illustrales the origin of the atoms of the purine ring, is noteworthy. The mechanism of several antimetabolites used in cancer chemotherapy and as antibiotics is based on their action on purine biosynthesis. Furthermore, a knowledge of the mechanism of action of allopurinol, a drug commonly used in the treatment of gout, requires an understanding of the pathway for purine biosynthesis and the reutilization (salvage) of existing purine metabolites. We will consider the pathway for purine biosynthesis step by step to gain an understanding of the process. Only experts, however, know it by rote.