cFos

c-Fos is a cellular proto-oncogene that encodes a member of the Fos family of basic leucine zipper (bZIP) transcription factors. The c-Fos protein forms heterodimers with members of the Jun family to create the activator protein 1 (AP-1) transcription factor, which regulates a wide range of target genes involved in proliferation, differentiation, and survival. The FOS gene is classified as an immediate early gene, meaning its transcript and protein are rapidly induced by stimulation without requiring new protein synthesis.

In neurons, c-Fos is rapidly upregulated in response to synaptic activity, growth factors, and stress. Its expression is commonly used as a marker of neuronal activation because it rises within minutes after stimuli and typically returns to baseline within a few hours. This transient induction is driven by signaling pathways such as calcium influx through NMDA receptors and MAP kinase cascades, which converge on the c-Fos promoter and AP-1–binding sites.

Molecularly, c-Fos contains a transactivation domain and a basic leucine zipper domain that mediates dimerization and DNA binding. The AP-1 complex binds DNA at specific motifs such as the TPA-responsive element, coordinating the transcription of genes that contribute to cellular responses to external cues.

In cancer biology, FOS is considered a proto-oncogene; its deregulated expression or genetic alterations can contribute to tumor development or progression in certain contexts, often in coordination with other oncogenic events such as Jun proteins or Ras signaling. However, c-Fos overexpression is not universally transforming and its effects are context-dependent.

c-Fos protein is relatively short-lived and regulated by ubiquitin-mediated degradation. It is commonly detected by immunohistochemistry to infer cellular activity, or by measuring FOS transcripts in studies of gene regulation.