UROS and cutaneous porphyria: For example, congenital erythropoietic porphyria (CEP [MIM 263700]) is an autosomal recessive disorder characterized by a deficiency in the enzymatic activity of uroporphyrinogen III synthase (UROS; EC 4.2.1.75), the fourth enzyme of the heme biosynthetic pathway (Supplementary Figure 1).