E4BP4, a mammalian basic leucine zipper (bZIP) transcription factor, was first identified through its ability to bind and repress viral promoter sequences. Subsequently, E4BP4 and homologues in other species have been implicated in a diverse range of processes including commitment to cell survival versus apoptosis, the anti‐inflammatory response and, most recently, in the mammalian circadian oscillatory mechanism. In some of these cases at least, E4BP4 appears to act antagonistically with members of the related PAR family of transcription factors with which it shares DNA‐binding specificity. This diversity of function is mirrored by the regulatory pathways impinging on E4BP4, which include regulation by ras via the lymphokine IL‐3 in murine B‐cells, by thyroid hormone during Xenopus tail resorption, by glucocorticoids in murine fibroblasts and by calcium in rat smooth muscle cells. This article will cover the unfolding role/s of and regulation of E4BP4, E4BP4‐like proteins and PAR factors in species as diverse as mouse and C. elegans. BioEssays 24:1023–1029, 2002. © 2002 Wiley‐Periodicals, Inc.