Basic leucine zipper transcription factor ATF-like (BATF), BATF2 and BATF3 belong to the activator protein 1 (AP-1) family of transcription factors, which regulate numerous cellular processes. Initially, BATF family members were thought to function only as inhibitors of AP-1-driven transcription, but recent studies have uncovered that these factors have unique, non-redundant and positive transcriptional activities in dendritic cells, B cells and T cells. The question of how BATF and BATF3 — which lack a transcriptional activation domain, unlike the AP-1 factors FOS and JUN — can exert unique positive transcriptional specificity has now been answered by the discovery that BATF molecules interact with members of the interferon-regulatory factor (IRF) family. The capacity of the BATF leucine zipper regions to mediate dimerization with AP-1 factors and also to define cooperative interactions with heterologous factors explains both the positive transcriptional activity of BATF proteins and how they activate distinct sets of target genes compared with FOS.