We previously reported the isolation and characterization of a cDNA clone, I-309, that encodes a small secreted protein produced by activated human T lymphocytes. This protein is structurally related to a large number of recently identified proteins that are secreted upon cellular activation. In this report we describe the isolation and characterization of the gene encoding I-309. The genomic organization is essentially identical to that found in the genes encoding the structurally similar proteins TCA-3, hJE/MCP-1, and mJE, strengthening the hypothesis that these genes are evolutionarily related. The region of the I-309 gene 5' of the mRNA cap site exhibits extensive nucleotide sequence homology with the same region of the murine gene TCA-3, providing additional evidence that I-309 and TCA-3 are likely to be homologs. Finally, panels of rodent-human somatic cell hybrids were used to map the I-309 gene to human chromosome 17. In conjunction with recent mapping data from other laboratories, this result suggests the presence of a cluster of related genes on this chromosome.