FOS and viral infectious disease: AP‐1 (activator protein 1) transcription factor is a heterodimeric protein composed of c‐Fos and c‐Jun. It regulates gene expression in response to a variety of stimuli, including cytokines, growth factors, stress, and bacterial and viral infections. AP‐1 regulates cell differentiation, proliferation, and apoptosis. In some cells AP‐1 plays a proapoptotic role by regulating the expression of a specific subset of target genes that promote efficient apoptotic cell death following mitotic arrest.