The cutaneous photosensitivity of PCT results from abnormally low hepatic URO-D activity.[1] Viewed stepwise, URO-D is the fifth of eight enzymes that participate in the heme biosynthetic pathway, and its function is to catalyze the conversion of uroporphyrinogen III to coproporphyrinogen III by the sequential removal from the tetrapyrrole of four carboxyl groups. Here, UROD is linked to porphyria cutanea tarda.