The human protooncogene JUN is the putative transforming gene of avian sarcoma virus 17, and it encodes a protein which is highly homologous to the viral protein. cJun (previously known as the Fos binding protein p39) and c Fos form a complex in the nucleus. AP 1 (activating protein 1) 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. AP 1 proteins, mostly the Jun group, regulate the expression and function of cell cycle regulators such as Cyclin D1, p53, p21 (cip1/waf1), p19 (ARF) and p16. Fos and Jun proto oncogene expression is induced transiently by a variety of extracellular stimuli associated with mitogenesis, differentiation processes or depolarization of neurons. JUN has been mapped to 1p32 to p31, a chromosomal region involved in both translocations and deletions in human malignancies.