The human protooncogene c-Jun is the putative transforming gene of avian sarcoma virus 17, and it encodes a protein which is highly homologous to the viral protein. c-Jun and c-Fos form a complex in the nucleus. AP1 is a collective term referring to these dimeric transcription factors composed of Jun, Fos or ATF subunits that bind to a common DNA site, the AP1 binding site.