ALA synthase, localised within the inner mitochondrial membrane, has been identified as the rate limiting enzyme of hepatic haem biosynthesis, subject to negative feedback control by haem. Liver, however, is a minor haem forming tissue as erythroblasts synthesise 85% of the total daily haem and are the cells most sensitive to haem deficiency. Within developing erythroid cells haem is required both for its specific complexing with globin and for the co-ordinate regulation of erythroblast metabolism and differentiation.