The immunoglobulin superfamily

lrnmunoglobulin polypeptide chains are composed of homology units. Related units are found in a large set of non-immunoglobulin proteins. Thus all these proteins, belonging to several functionally diversified families, can be grouped into, a single superfamily, the immunoglobulin superfamily, and based exclusively on structural homology. Such homology between members of different families is limited as a rule to the protein structure, homology being virtually nonexistent at the DNA level, except for a very general principle of organization (each homology unit is usually encoded by a single exon). However, within individual families, DNA sequence homology is found to pseudogenes and to gene sequences which are not known to be expressed. These gene sequences must be regarded as members of the immunoglobulin superfamily. since they may play or may have played important roles in its evolution. Any unifying hypothesis is bound to introduce some degree of distortion to the experimental data, and this may be very significant when. as in the case of the immunoglobulin superfamily, the unifying criterion can often be only partially verified. However, such a hypothesis is justified in so far as it makes specific predictions which are testable. The analysis o f how variations of a basic structural module are achieved during evolution through the operation of a variety of genetic mechanisms and of how such variations can serve different functional purposes is clearly of both great theoretical and practical interest.