The transcription factor AP-1 consist of a variety of dimmers composed of members of the Fos, Jun and ATF families of proteins. The activity of individual AP-1 components is regulated at multiple levels and participate in the regulation of a variety of cellular process including cell proliferation, apoptosis, differentiation and oncogenesis. Mice lacking c-Jun die at midgestation due to liver and heart defects. In addition to AP-1 proteins, c-Jun is capable of interacting with a number of proteins and form a heterotrimer, most notably with the Nuclear Factor of Activated T cells family (NFAT1-4). The binding of AP-1 and NFAT produce a strong coooerative complex cooperation between these unrelated families of transcriptions factors has been implicated in many biological processes in immune cells, but the role in extra-immune tissues remains to be understood. The objectives of this proposal is to clarify the importance of the AP-1/NFAT cooperation in lymphoid cells and to uncover novel biological processes dependent on c-Jun/NFAT interaction in extra-immune tissues, as well as, the requirement of the transactivation activity of c-Jun in these processes. I will generate two transgenic mices with the c-Jun allel mutated.