[Disorders of hemoglobin synthesis (exclusive of iron deficiency)]

Hemoglobinopathies in the strict sense most often involve single amino acid substitutions. The clinical consequences, if any, are anemia, erythrocytosis and met-hemoglobinemia. In the thalassemias, which are characterized by microcytosis, unbalanced synthesis of normal globin chains exists and disease severity is primarily determined by the amount of relative overproduction of the unaffected chain: thus, the unstable chain tetramers precipitate and lead to ineffective erythropoiesis and hemolysis. The sideroblastic anemias are a heterogeneous group of pathological conditions, both clinically and pathogenetically. Heme synthesis appears to be regularly disturbed, but this is probably not the sole defect. Other alterations of globin and heme synthesis are listed and briefly discussed.