A group of rare inherited ENZYME diseases in which disorders of the metabolic pathways leading to the synthesis of HAEM cause excessive production of haem precursors called PORPHYRINS by the bone marrow or liver. The excess porphyrins in the blood mainly affect the skin, causing PHOTOSENSITIVITY, or the central nervous system, producing neuropsychiatric symptoms. Excess porphyrins can be detected in blood, urine and faeces. Usually porphyrias are genetically determined, but one form is due to alcoholic liver disease. The commonest form, porphyria cutanea tarda, affects up to 1 in 5,000 people in some countries. The British king, George III, suffered from porphyria, a disorder unrecognised in the 18th century.