The erythropoietic porphyrias.

There are two well-characterized disorders of porphyrin-heme metabolism in which the bulk of the porphyrins that accumulate in excess are formed chiefly within juvenile erythroid elements of the bone marrow. The first is most often termed congenital erythropoietic porphyria, and the second is most often termed erythropoietic protoporphyria. The former is a rare disorder, whereas the latter is one of the two most common porphyrias (along with porphyria cutanea tarda) and has a good probability of being encountered in general dermatologic practice. Both are determined genetically, cause cutaneous photosensitivity and systemic complications of importance, and are amenable to various forms of therapy.