The cJun proto-oncogene was first identified as the cellular homolog of the avian sarcoma virus vjun oncogene. JunB and JunD have been shown to be almost identical to cJun in their C terminal regions, which are involved in dimerization and DNA binding, whereas their N terminal domains, which are involved in transcriptional activation, diverge. JunB is a transcription factor involved in regulating gene activity following the primary growth factor response. It binds to the DNA sequence 5'-TGA[CG]TCA-3'.The protein encoded by this intronless gene is a member of the JUN family, and a functional component of the AP1 transcription factor complex. It has been proposed to protect cells from p53-dependent senescence and apoptosis. Alternate translation initiation site usage, including non-AUG codons, results in the production of different isoforms.