AP-1 transcription factor known to play a role in cell proliferation and neuronal activation, it is also involved in apoptosis of cells in response to stress, DNA damaging agents or lack of survival signals. AP-1 DNA binding complex is not a single transcription factor but a dimer consisting of members of Fos and Jun families. In this review, we discuss evidence that composition of the AP-1 complex is different under various physiological and pathophysiological conditions. Furthermore, we describe biochemical properties of Fos and Jun proteins that may explain the ability of this transcription factor to activate different sets of genes in response to different stimuli. We propose a hypothesis that AP-1 might contribute to distinct biological processes because an activation of specific signaling pathways results in changes of AP-1 composition and/or phosphorylation status and modulates its transactivating potential towards different promoters.