Molecular analysis of normal and abnormal human globin genes and their gene products has recently provided information on the precise genetic events that result in hemoglobinopathies. In the case of structurally abnormal hemoglobins, the following mechanisms can be invoked: single nucleotide base substitutions leading to amino acid replacement or chain termination variants; nucleotide deletions (or additions) leading to deletion and frameshift variants; and nonhomologous crossing over leading to the production of fused globin chains. The molecular basis of the thalassemia syndromes, disorders characterized by absent or decreased synthesis of α- or β-globin chains, is quite heterogeneous. In some cases globin gene deletions have been demonstrated; whereas in others there is probably either a defect in globin gene transcription or a defect in nuclear globin messenger RNA (mRNA) processing, mRNA transport or globin mRNA stability. In one form of β°-thalassemia a nonsense mutation has recently been demonstrated, and other cases are also associated with some as yet undetermined functional abnormality of β-globin mRNA.