Interleukin 2 (IL-2) is produced by optimally stimulated T cells and is the major growth factor for T cells. Quiescent T cells do not synthesize IL-2, but upon stimulation, IL-2 is quickly expressed due to the coordinate binding of transcription factors within a 300 base pair promoter. Activation of the transcription factors involved in IL-2 transcription is highly dependent on proper stimulation through the T cell receptor (TCR) as well as a co-stimulatory molecule such as the CD28 receptor. TCR-induced signals are necessary to induce the binding of most, but not all, factors to the IL-2 promoter. CD28 stimulation synergizes with the TCR to activate many of these factors, but is also responsible for the induced binding of a novel complex to a region of the IL-2 promoter known as the CD28 responsive element (CD28RE). Determination of the proteins which function at this site was investigated. Functional studies led to the discovery that the CD28RE forms a composite element with a proximal AP-1 site. Together, the CD28RE/AP-1 sequences bind a complex that contains c-Rel, c-Fos, c-Jun, and the novel protein p21 SNFT in Jurkat T cells. Further analysis of p21SNFT indicates that it inhibits IL-2 promoter activity due to its ability to repress AP-1-associated enhancers required for transcription. Functional and binding studies using nuclear extracts and purified proteins suggest that p21 SNFT competes with Fos proteins for Jun binding, thereby effectively decreasing the numbers of transcriptionally active AP-1 heterodimers that form. While much is known about transactivation requirements for IL-2 expression, less is known about the inhibitory processes that shut down or prevent IL-2 transcription. Therefore, the identity of a novel protein that represses IL-2 promoter activity may help in our understanding of the mechanisms involved in the silencing of the IL-2 gene in situations such as in sub-optimally stimulated T cells, anergic T cells, or during T cell differentiation.