Fos and Jun dimerize to form Activator Protein 1 (AP1), a transcriptional factor that binds to the 12-O-tetradecanoylphorbol 13 acetate (TPA) response element (TRE) of several cellular and viral genes including human collagenase, metallothionein IIa, stromelysin, interleukin 2, SV40 and polyoma. Fos and Jun contain the 'leucine-zipper' motif that allows for dimerization and an adjacent basic domain required for biological activity. The functionally active form of Fos is in a heterodimer