FOS (c-Fos, FBJ murine osteosarcoma viral oncogene homolog) is a component of the DNA-binding AP-1 complex together with c-Jun that regulates gene expression and is upregulated in numerous cancers including osteocarcinoma, endometrial cancer, and breast cancer. FOS is involved in the proliferation, differentiation, survival, and angiogenesis of cells, and deregulation of FOS can induce invasion and metastasis via destabilization of these various processes. Separately, FOS is used as a marker of neuronal activity, due to FOS being upregulated after neurons fire action potentials. Its activity is relevant to drug addiction research, as it is upregulated in the prefrontal cortex and nucleus accumbens following use of methamphetamines and psychoactive drugs. In immunohistochemistry, FOS has positive nuclear and cytoplasmic staining in most tissues throughout the body. References: Histol. Histopathol. 15 (3): 921–8. PMID 10963134; Br. J. Cancer. 99 (8): 1269–75, PMID: 18854825; Proc. Natl. Acad. Sci. U.S.A. 87 (17): 6912–6, PMID: 2118661; Biol. Psychiatry. 53 (4): 275–83, PMID: 12586446; Journal of Neuroscience Methods. 29 (3): 261–265, PMID: 2507830;