The production of the nuclear oncoproteins Fos and Jun is rapidly induced in response to extracellular signals. In the nucleus, the two proteins combine to form a tight complex via leucine zipper domains. The resulting Fos-Jun heterodimer can bind to the TPA-responsive element (TRE) by way of a novel, highly basic motif and can activate the transcription of TPA-responsive genes. The existence of several Fos- and Jun-related proteins with dimerization and DNA binding properties similar to Fos and Jun suggests that these two oncoproteins may be part of a network of related but functionally distinct transcription factors.