UROS, also known as Uroporphyrinogen III synthase, is an enzyme involved in the fourth step of porphyrin metabolism, involved in the conversion of hydroxymethyl bilane into uroporphyrinogen III. Defects in this protein can cause molecular lesions that lead to the autosomal recessive Gunther disease, also known as congenital erythropoietic porphyria (CEP). Recombinant human UROS protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography.