Cytochrome b5 reductase 2, also known as CYB5R2, is involved in desaturation and elongation of fatty acids, cholesterol biosynthesis, drug metabolism, and, in erythrocyte, methemoglobin reduction. This protein is responsible for NADH-dependent lucigenin chemiluminescence in spermatozoa by reducing both lucigenin and 2-[4-iodophenyl]-3-[4-nitrophenyl]-5-[2,4-disulfophenyl]-2H tetrazolium monosodium salt (WST-1). Recombinant human CYB5R2 protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography techniques.