ALAS1 and porphyria: This step is catalyzed by the enzyme ALA synthase (ALAS), which has two subtypes—ALAS1, the ubiquitous one, encoded on chromosome 3, and ALAS2, erythroid-specific, encoded on chromosome X. In the erythropoietic tissue, regulation of heme synthesis is influenced by erythroid differentiation and erythropoietin and iron availability, while in the liver, ALAS1 is under negative feedback regulation by the intracellular heme pool.3 This regulatory mechanism is the basis of strategies in treatment and management of porphyria patients.