The activator protein 1 transcription factor. The dimeric activator protein 1 (AP-1) transcription factor is composed of Jun and Fos proteins. Jun proteins form homodimers or heterodimers with Fos proteins through their leucine-zipper domains. The different dimer combinations recognize different sequence elements in the promoters and enhancers of target genes. Only the classic TPA-responsive element with the consensus sequence TGACTCA is shown. The AP-1 dimers recognize the specific response elements via the basic domain that is adjacent to the leucine-zipper domain and represent an α-helical structure. Among the target genes of AP-1 are important regulators of cell proliferation, differentiation and apoptosis. Some AP-1 targets are implicated in pathogenic processes such as S100a8 and S100a9. Positively regulated (+), negatively regulated (-), or positively and negatively regulated (+/-) depending on the AP-1 dimer composition.