Nuclear phosphoprotein which forms a tight but non-covalently linked complex with the JUN/AP-1 transcription factor. FOS 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. In growing cells, may activate phospholipid synthesis (By similarity). Heterodimer. Interacts with MAFB. In quiescent cells, may be present in very small amounts in the cytosol. Following induction of cell growth, first localizes to the endoplasmic reticulum and later to the nucleus (By similarity). Expression increases upon a variety of stimuli, including growth factors, cytokines, neurotransmitters, polypeptide hormones, stress and cell injury. May be Tyr-phosphorylated in quiescent cells and dephosphorylated upon cell growth induction. Belongs to the bZIP family. Fos subfamily.