The irreversible synthesis of cytosolic 5'-phosphoribosyl-5-aminoimidazole (AIR) from 5'-phosphoribosylformylglycinamidine (FGAM), accompanied by the conversion of ATP to ADP and orthophosphate, is catalyzed by the phosphoribosylaminoimidazole synthetase domain of the trifunctional protein, "phosphoribosylglycinamide formyltransferase, phosphoribosylglycinamide synthetase, phosphoribosylaminoimidazole synthetase" (GART) (Aimi et al. 1990). The active form of the protein is cytosolic and may co-localize with other enzymes of de novo IMP biosynthesis under some metabolic conditions (Gooljarsingh et al. 2001; An et al. 2008).