The nuclear proto-oncogenes, c-fos and c-jun, are induced in response to a diverse array of extracellular stimuli. Their protein products, Fos and Jun, form a heterodimeric complex that interacts with the DNA regulatory element known as the AP-1 binding site. Protein dimerization occurs via a parallel interaction of leucine zipper domains and is required for DNA binding. In addition to the leucine zipper, DNA binding requires two clusters of basic amino acids adjacent to the leucine zipper domains of both Fos and Jun. The leucine zipper and DNA-binding regions are highly conserved among the c-fos and c-jun families of related inducible genes. Thus, multiple protein complexes can be formed that may interact with AP-1 binding sites in numerous genes to affect gene expression in response to environmental signals.