The porphyrias are a group of rare disorders that affect heme biosynthesis (the formation of chemical compounds by living organisms). Heme is an essential component of hemoglobin (the iron-containing pigment of the blood that carries oxygen from the lungs to the tissues) as well as many enzymes throughout the body. Biosynthesis of heme is a process that starts with simple molecules and ends with a large, complex heme molecule. Each step of the biosynthesis pathway is directed by its own protein, called an enzyme. As a heme precursor molecule moves through each step, an enzyme modifies it in some way. If the precursor is not modified, it cannot proceed to the next step.

Due to a defect in one of the enzymes of the heme biosynthesis pathway, protoporphyrins or porphyrin (heme precursors) are prevented from proceeding further along the pathway. Instead, precursors accumulate at the stage of the...