Uroporphyrinogen III synthase（UROS) is an enzyme involved in the 4th step of porphyrin metabolism and in the conversion of hydroxymethyl bilane into uroporphyrinogen III. Defects in the UROS protein can cause molecular lesions which lead to the autosomal recessive Gunther disease， otherwise known as congenital erythropoietic porphyria（CEP).
