The JunB transcription factor, also known as JunB proto-oncogene, JunB oncogene, transcription factor JunB, and transforming protein JunB, is a basic region leucine zipper DNA-binding protein, and a member of the Jun family of proteins, which also includes c-Jun and JunD. Jun proteins can homodimerize or heterodimerize with other Jun family members, or heterodimerize with Fos family members (cFos, FosB, Fra-1, and Fra-2) to form the activator protein 1 (AP-1) transcription factor. The various dimers differ in their ability to transactivate AP-1-dependent genes. Jun B8 and JunD1 are almost identical to c-Jun in their C-terminal regions, which are involved in dimerization and DNA binding, whereas their N-terminal domains, which are involved in transcriptional activation, diverge. JunB and C-Jun antagonistically control cytokine-regulated mesenchymal-epidermal interaction in skin. Cyclin A is a direct transcriptional target of JunB driving cell proliferation. JunB is an important regulator of erythroid differentiation and is also important for the development of Th2 effector cells, normal Th2 cytokine expression pattern, and a complete Th2- dependent immune response in mice JunB expression levels are increased in human colorectal adenocarcinoma specimens as compared to normal mucosa.