The heme biosynthetic pathway occurs in practically all cell types. Heme is used to form different hemoproteins in a variety of organelles. Within hepatic and erythroid cells, for example, considerable quantities of heme are used as prosthetic groups for cytochrome P450 and hemoglobin, respectively. The regulation of heme biosynthesis has been under investigation for many years. In non-erythroid cells, the rate of heme biosynthesis depends on 5-aminolevulinate synthase (ALAS). This enzyme is largely controlled by the concentration of heme itself. In contrast, heme biosynthesis in erythroid cells is regulated by iron availability. Erythroid cells possess a 5-aminolevulinate synthase isozyme (e-ALAS) containing an iron responsive element (IRE).