The purine ribonucleotide inosine 5'-monophosphate (IMP) is assembled on 5-phospho-alpha-D-ribose 1-diphosphate (PRPP), with atoms derived from aspartate, glutamine, glycine, N10-formyl-tetrahydrofolate, and carbon dioxide. Although several of the individual reactions in this sequence are reversible, other irreversible steps drive the pathway in the direction of IMP synthesis in the normal cell. All of these reactions are thus annotated here only in the direction of IMP synthesis. IMP is the starting point for the biosynthesis of other purine nucleotides (Zalkin and Dixon 1992).