Uroporphyrinogen-III Synthase is an enzyme which belongs to the uroporphyrinogen-III synthase family. Uroporphyrinogen-III Synthase is ubiquitous and it is involved in Porphyrin metabolism. Porphyrins act as cofactors for a multitude of enzymes that perform a variety of processes within the cell such as Methionine synthesis (Vitamin B12) or oxygen transport (Heme). Uroporphyrinogen-III Synthase can catalyze cyclization of the linear Tetrapyrrole, Hydroxymethylbilane, to the Macrocyclic Uroporphyrinogen III, the branch point for the various sub-pathways leading to the wide diversity of Porphyrins. Defects in Uroporphyrinogen-III Synthase are the cause of Congenital Erythropoietic Porphyria (CEP).