FUNCTION: Nuclear phosphoprotein which forms a tight but non-covalently linked complex with the JUN/AP-1 transcription factor. Has a critical function in regulating the development of cells destined to form and maintain the skeleton. It is thought to have an important role in signal transduction, cell proliferation and differentiation. SUBUNIT: Heterodimer. Interacts with DSIPI; this interaction inhibits the binding of active AP1 to its target DNA. Interacts with MAFB. SUBCELLULAR LOCATION: Nucleus. INDUCTION: C-fos expression increases upon a variety of stimuli, including growth factors, cytokines, neurotransmitters, polypeptide hormones, stress and cell injury. SIMILARITY: Belongs to the bZIP family. Fos subfamily. SIMILARITY: Contains 1 bZIP domain