The second type is further divided into classical erythropoietic protoporphyria (EPP) caused by ferrochelatase (FECH) loss-of-function mutation, and X-linked dominant protoporphyria (XLPP) caused by aminolavulinic acid synthetase 2 (ALAS2) gain-of-function mutation [83]. Here, FECH is linked to erythropoietic protoporphyria.