The T-cell-specific adapter protein (TSAd), encoded by the SH2D2A gene, has been implicated in modulation of proximal signaling events as well as in transcriptional regulation in human T cells. We have isolated its rat homologue ( rSH2D2A) from an NK cell cDNA library and mapped the corresponding gene to chromosome 2 with a hamster-rat radiation hybrid cell panel. rSH2D2A encodes a 376 amino acid protein (rTSAd) which shows greater homology to mouse than human TSAd. In rats, rTSAd was specifically expressed by NK cells and T cells but not by other leukocytes tested. Similarly, in humans we observed abundant transcripts for TSAd in NK cells and T cells. The data suggest that TSAd may have a regulatory role in cellular activation of T and NK cells.