The human T cell antigen-receptor γ-chain, which is expressed on the surface of a subpopulation of CD3+ T lymphocytes, exhibits size polymorphism and varies in its ability to form disulfide bonds with a second polypeptide. Analysis of both genomic and complementary DNA clones encoding the human γ polypeptide shows differences in lengths of the coding portions of the two constant region genes, Cγ1 and Cγ2. A single second-exon segment is always present in the Cγ1 gene. Cγ2 alleles containing either duplicated or triplicated second-exon segments are present in the normal human population and are expressed as messenger RNAs. Furthermore, a cysteine residue, encoded by the second exon of Cγ1 and probably involved in interchain disulfide bridging, is absent in all Cγ2 second-exon segments. These differences between Cγ1 and the two alleles of Cγ2 may explain the variability in molecular weight and disulfide bonding of γ molecules expressed in different cells.
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