GMPR, also known as guanosine monophosphate reductase, catalyzes the irreversible NADPHdependent deamination of GMP to IMP. It functions in the conversion of nucleobase, nucleoside and nucleotide derivatives of G to A nucleotides, and in maintaining the intracellular balance of A and G nucleotides. Recombinant human GMPR protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography.