STAT4 is a transcription factor activated in response to IL-12, and is involved in Th1 cell development. The molecular mechanisms controlling the transcription of the STAT4 gene are however, unclear. Sequence comparison of the 5' flanking regions of human, mouse and pufferfish (Fugu rubripes) Stat4 genes revealed a high frequency of Ikaros (Ik) binding elements in all three species. We then investigated the role of Ik binding elements in the human STAT4 promoter using Jurkat T cells. Transactivation, electrophoretic mobility shift assay and RNA interference-mediated gene knockdown experiments revealed that Ik is involved in the regulation of STAT4 in human T cells.