The transcription factor AP-1 (activator protein-1) is involved in cellular proliferation, transformation and death. AP-1 and nuclear factor B (NF B) can be specifically targeted to prevent cancer induction in mouse models. AP-1 can be produced by 18 different dimeric combinations of proteins from the Jun c-Jun, JunB and JunD and Fos (c-Fos, FosB, Fra-1 and Fra-2) families, including Jun homodimers and Jun-Fos heterodimers. The Jun and Fos proteins contain a basic-region leucine zipper (bZIP) domain, and are capable of binding to other bZIP proteins including those from the ATF, MAF, CNC and C/EBP (CCAAT/enhancer-binding protein) subfamilies. Jun-Jun and Jun-Fos dimers bind with highest affinity to the 12-O-tetradecanoylphorbol-13-acetate (TPA) response element (TRE) TGA(C/G)TCA, although many other AP-1-like sites have been reported. Binding to any of these sites can be tissue-specific, or affected by neighboring sequences, and dependent upon interactions with other transcription factors or cofactors. Jun and Fos proteins can also dimerize with other bZIP proteins, allowing them to target other DNA binding sites, such as the cAMP response element (CRE), the antioxidant response elements (ARE), and half-sites composed of half of a TRE site and half of a MAF- or CNC-binding site. In addition, AP-1 proteins can interact with other proteins, including the p65 subunit of NF-B, CBP (CRE-binding-protein-binding protein) (p300), SMAD-3 and -4, and the retinoblastoma protein (see 5 for a more complete list), further increasing the combinatorial potential of Jun and Fos proteins. AP-1 regulates a variety of cellular processes, including proliferation, differentiation and apoptosis, and contributes to both basal and stimulus-activated gene expression. It is activated by growth factors, hormones, stress, cytokines, ROS and ultraviolet radiation. Activation occurs both transcriptionally and post-translationally, and is signaled predominantly through the mitogen-activated protein kinase (MAPK) cascade. The combinatorial diversity of AP-1 proteins and other interacting factors appears to influence how specific cell types respond to a stimulus.