The reversible synthesis of inosine 5'-monophosphate from 5'-phosphoribosyl-5-formaminoimidazole-4-carboxamide (FAICAR) is catalyzed by the IMP cyclohydrolase activity of the bifunctional 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase/IMP cyclohydrolase enzyme. This cytosolic protein occurs primarily as a dimer (Rayl et al. 1996; Vergis et al. 2001) and may further associate with other enzymes of de novo IMP biosynthesis under some metabolic conditions (An et al. 2008).