Erythropoietic protoporphyria (EPP) is a painful photodermatosis without blisters caused by inborn errors of the haem biosynthetic pathway due, in the majority of patients, to a deficient activity of the enzyme ferrochelatase (ferrohaemprotolyase, haem synthetase, ferrohaem-protolyase, EC4.99.1.1, FECH (EPP), (MIM 177000) or caused, in about 2% of patients, by a gain of activity of the erythroid specific aminolevulinic acid synthase 2 (ALAS; EC 2.3.1.27). The gene discussed is FECH; the disease is autosomal erythropoietic protoporphyria.