Transcription factor involved in regulating gene activity following the primary growth factor response. Binds to the DNA sequence 5'-TGA[GC]TCA-3'. Heterodimerizes with proteins of the FOS family to form an AP-1 transcription complex, thereby enhancing its DNA binding activity to an AP-1 consensus sequence and its transcriptional activity (By similarity). . This is the function of JUNB (JunB proto-oncogene, AP-1 transcription factor subunit, ENSG00000171223).