The irreversible transfer of a formyl group to 5'-phosphoribosyl-5-aminoimidazole-4-carboxamide (AICAR), to yield 5'-phosphoribosyl-5-formaminoimidazole-4-carboxamide (FAICAR), is catalyzed by the phosphoribosylaminoimidazolecarboxamide formyltransferase 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).