Heme is synthesized in all cell types in aerobic organisms. Hydroxy-methylbilane synthase (HMBS) and uroporphyrinogen III synthase (UROS) catalyze two consecutive reactions in the heme biosynthetic path-way, generating the first linear and the first cyclic tetrapyrroles, respec-tively. Each of the HMBS and UROS genes contains the two separate promoters that generate ubiquitous and erythroid-specific mRNAs.