The hinge region of the immunoglobulin molecule is responsible for antigen-binding and cross-linking reactions, varying the distance between the two antigen-binding sites. As the amino acid sequence of the hinge region is identical among immunoglobulin molecules of the same (sub)class, it has been regarded as a constant region. By comparison of the nucleotide sequences among primate C alpha genes, it is clear that there is a wide variety of length among the hinge regions of hominoid C alpha genes, which basically consist of tandem repeats of a 15 base-pair sequence. This reiterated structure probably facilitates rapid evolutionary changes in the length of the hinge region. The hinge region of the Old World monkey C alpha gene has a non-reiterated structure whose nucleotide sequence is quite different from those of the hominoid C alpha genes, although its surrounding region is conserved during evolution. This unusual hypervariability reveals that the hinge region has evolved as a semi-variable region in contrast to its constant character from an ontogenic viewpoint.