Erythropoietic protoporphyria (EPP) is a genetic disorder. It results from mutations in genes involved in the haem production pathway, such as ferrochelatase and delta-aminolevulinate synthase 2. The condition results in excessive amounts of protoporphyrin IX in the skin, bone marrow, blood plasma and red blood cells. EPP is a cutaneous porphyria. The major symptom is phototoxicity (a chemical reaction underneath the skin) caused by sunlight and artificial light emitted along the visible spectrum above 400 nanometres. The skin can rapidly become severely painful, swollen, itchy and red, and skin erosions can also occur. A phototoxic reaction typically lasts between 2 days and 3 days. But it can last 10 or more days, with severe pain and loss of sleep. These symptoms, along with persisting anxiety and social isolation because of sun and light avoidance, can have a profound effect on quality of life. Over time, light exposure can cause thickening of the skin on the knuckles and scarring on the face. A small proportion of people with EPP may have important complications related to liver and gallbladder function.