The humoral immune system antigen-binding proteins (immunoglobulins) are disulphide-linked heterodimers of light and heavy chains. The gene for the variable region which determines antigen specificity is assembled when one member from each of the dispersed clusters of variable (V) gene segments, diversity (D) elements (for the heavy chains only) and joining (J) segments rearrange and fuse during B-cell development (reviewed in ref. 1). Short recognition sequences adjacent to these elements appear to be involved in the recombination process. The cellular immune system antigen recognition proteins are receptors on the surface of T cells, which are composed of disulphide-linked α-chains and β-chains, each of which has a variable and constant region2-4. Recently, cDNA clones of the β-chain5 mRNA have been isolated6,7; the genomic arrangement is very similar to immunoglobulin genes with multiple Vβ genes8, and two clusters of Jβ segments, each of which is upstream from a constant-region gene segment9. The Vβ and J β segments have adjacent recombinational recognition sequences like the immunoglobulin elements. However, ∼10 nucleotides of the cDNA clones between the Vβ and Jβ regions were not present in the corresponding genomic elements and may have been due to intervening Dβ segments8,10. Here we describe a diversity element (Dβ1.1) in a region of high human-mouse homology about 650 bases 5′ to the first Jβ cluster. Two transcripts which include sequences upstream of Dβ1.1 are found in the human thymus. This region may have some other function besides providing the β-chain with a diversity segment.
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