Schematic representation of the purine nucleotide synthesis pathway in yeast. The solid thin lines represent the plasma membrane. ADP: adenosine-5'-diphosphate; AMP: adenosine-5'-monophosphate; ATP: adenosine-5'-triphosphate; GDP: guanosine-5'-diphosphate; GMP: guanosine-5'-monophosphate; GTP: guanosine-5'-triphosphate; IMP: inosine-5'-monophosphate; PRPP: 5-phosphoribosyl-1-pyrophosphate; SAMP: S-adenosine-5'-monophosphate; XMP: xanthosine-5'-monophosphate. Genes are shown in grey italic font and encode the following enzymatic activities: AAH1: adenine deaminase; ADE8: 5'-phosphoribosylglycinamide formyltransferase; ADE12: adenylosuccinate synthetase; ADE13: adenylosuccinate lyase; ADK1: AMP kinase; AMD1: AMP deaminase; APT1: adenine phosphoribosyltransferase; FCY2: purine cytosine permease; GUA1: GMP synthetase; GUK1: GMP kinase; HPT1: hypoxanthine-guanine phosphoribosyltransferase; IMD2, IMD3, and IMD4: IMP dehydrogenases (IMD1 is not indicated here because it is not expressed and is thought to be a pseudogene). Mycophenolic acid (MPA) inhibits IMP dehydrogenases.