In the fields of molecular biology and genetics, c-Fos is a proto-oncogene that is the human homolog of the retroviral oncogene v-fos. Fos proteins dimerize with Jun proteins (c-Jun, JunB, and JunD) to form Activator Protein-1 (AP-1), a transcription factor that binds to TRE/AP-1 elements and activates transcription. c-fos plays an important role in many cellular functions and has been found to be overexpressed in a variety of cancers. Expression of c-fos is an indirect marker of neuronal activity because c-fos is often expressed when neurons fire action potentials. Upregulation of c-fos mRNA in a neuron indicates recent activity.
