Mutations in the UROS gene cause the third form of erythropoietic protoporphyria, called congenital erythropoietic protoporphyria (CEP; OMIM: #263700); the partial loss of UROS activity leads to the incomplete metabolism of hydroxymethylbilane and the accumulation of non-physiologic porphyrin isomers in the bone marrow, erythrocytes, urine and other organs. Here, UROS is linked to erythropoietic protoporphyria.