The irreversible transfer of a formyl group to cytosolic 5-phosphoribosylglycinamide (GAR) to form 5'-phosphoribosylformylglycinamide (FGAR) is catalyzed by the phosphoribosylglycinamide formyltransferase domain of the trifunctional protein, "phosphoribosylglycinamide formyltransferase, phosphoribosylglycinamide synthetase, phosphoribosylaminoimidazole synthetase" (GART) (Aimi et al. 1990; Zhang et al. 2002). Fluoresence microscopy studies of cultured human cells have shown that GART is cytosolic and suggest that it may co-localize with other enzymes of de novo IMP biosynthesis under some metabolic conditions (Gooljarsingh et al. 2001; An et al. 2008).