Mutations in human ALAS2 yielding diminished ALAS activity or “loss-of-function” are associated with X-linked sideroblastic anemia (XLSA; MIM 300751) [7,8], while “gain-of-function” ALAS2 mutations are present in X-linked protoporphyria (XLPP; MIM 300752) [8,9,10,11,12]. The gene discussed is ALAS2; the disease is X-linked erythropoietic protoporphyria.