Delta beta thalassemia and hereditary persistence of fetal hemoglobin.

Delta beta Thalassemia and hereditary persistence of fetal hemoglobin (HPFH) constitute a heterogeneous group of disorders characterized by absent or reduced synthesis of adult hemoglobin (Hb A) and increased synthesis of fetal hemoglobin (Hb F). Coinheritance of these disorders with other beta chain hemoglobinopathies, such as beta thalassemia and the sickle cell (beta s) gene, can result in attenuation of the clinical severity of these hemoglobinopathies owing to the increased Hb F levels. The molecular basis of these disorders is quite heterogeneous and consists of both deletion and nondeletion types of mutations. The characterization of these molecular defects has provided new insights on the structure and function of important regulatory elements that are involved in the normal control of expression of the beta- and gamma-globin genes and in hemoglobin switching.