Porphyria cutanea tarda, the most common type of porphyria worldwide, is caused by a deficiency of uroporphyrinogen decarboxylase, a crucial enzyme in heme biosynthesis, which results in an accumulation of photosensitive byproducts, such as uroporphyrinogen, which leads to the fragility and blistering of sun-exposed skin. The gene discussed is UROD; the disease is porphyria.