Basic leucine zipper (bZIP) domain of the oncogene Fos (Fos): a DNA-binding and dimerization domainFos proteins are members of the activator protein-1 (AP-1) complex, which is mainly composed of Basic leucine zipper (bZIP) dimers of the Jun and Fos families, and to a lesser extent, the activating transcription factor (ATF) and musculoaponeurotic fibrosarcoma (Maf) families. The broad combinatorial possibilities for various dimers determine binding specificity, affinity, and the spectrum of regulated genes. The AP-1 complex is implicated in many cell functions including proliferation, apoptosis, survival, migration, tumorigenesis, and morphogenesis, among others. There are four Fos proteins: c-Fos, FosB, Fos-related antigen 1 (Fra-1), and Fra-2. In addition, FosB also exists as smaller splice variants FosB2 and deltaFosB2. They all contain an N-terminal region and a bZIP domain. c-Fos and FosB also contain a C-terminal transactivation domain which is absent in Fra-1/2 and the smaller FosB variants. Fos proteins can only heterodimerize with Jun and other AP-1 proteins, but cannot homodimerize. Fos:Jun heterodimers are more stable and can bind DNA with more affinity that Jun:Jun homodimers. Fos proteins can enhance the trans-activating and transforming properties of Jun proteins. bZIP factors act in networks of homo and heterodimers in the regulation of a diverse set of cellular processes. The bZIP structural motif contains a basic region and a leucine zipper, composed of alpha helices with leucine residues 7 amino acids apart, which stabilize dimerization with a parallel leucine zipper domain. Dimerization of leucine zippers creates a pair of the adjacent basic regions that bind DNA and undergo conformational change. Dimerization occurs in a specific and predictable manner resulting in hundreds of dimers having unique effects on transcription.