The human gene encoding the Ia-associated gamma (or invariant) chain was isolated by screening a genomic library in phage lambda with cDNA probes. The frequency of positive clones in the library, the overlapping restriction maps of the cloned fragments, and the patterns of genomic hybridization suggested that the gamma-chain gene exists as a single copy per haploid genome. The gene consists of 8 exons, spanning approximately 12 kilobases of DNA. All exon sequences were in an open reading frame, contained appropriate splice junction sequences, and encompassed the entire sequence of full-length gamma-chain mRNA, suggesting that the gene we isolated is most likely functional. Furthermore, "CAAT"-type and "TATA"-type promoter sequences were found at the expected positions upstream from the proposed cap site. The organization of the gamma-chain gene has none of the distinctive features of the immunoglobulin superfamily of genes, of which Ia alpha and beta chains are members. Therefore, the evolutionary origins, and perhaps the functions, of the Ia gamma chains are distinct from those of the other two Ia subunits alpha and beta. Despite the unrelatedness of these genes, consensus sequences found approximately 150 base pairs upstream from all the Ia alpha- and beta-chain genes sequenced to date were also found in analogous positions in the gamma-chain gene, suggesting a possible role in the coregulation of expression of these genes.