The phosphoribosylaminoimidazole carboxylase activity of cytosolic PAICS (multifunctional protein ADE2) catalyzes the reaction of 5'-phosphoribosyl-5-aminoimidazole (AIR) and CO2 to form 5'-phosphoribosyl-5-aminoimidazole-4-carboxylate (CAIR). Chicken PAICS protein has been purified and its catalytic properties analyzed in vitro. Its aminoterminus is modified by deletion of the first two residues encoded by the PAICS gene (Firestine and Davisson 1994; Firestine et al. 1994). The native chicken protein behaves as a large multimer on gel filtration; it is inferred to be a octamer by analogy to the human protein (Li et al. 2007). The subcellular location of the chicken enzyme is inferred from that of its better-characterized mammalian counterparts.