PAICS, also known as phosphoribosylaminoimidazole carboxylase, is an enzyme involved in nucleotide biosynthesis and in particular in purine biosynthesis. It catalyzes the conversion of 5'-phosphoribosyl-5-aminoimidazole(AIR) into 5'-phosphoribosyl-4-carboxy-5-aminoimidazole (CAIR) as described in the reaction. Recombinant human PAICS protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography.