FOS is a proto-oncogene encoding a nuclear phosphoprotein that functions as a core component of the AP-1 transcription factor complex 1. FOS heterodimerizes with JUN family proteins to form AP-1, which binds symmetrical DNA half-sites to regulate gene transcription [UniProt]. The protein also participates in TGF-β signaling by forming multimeric SMAD3/SMAD4/JUN/FOS complexes at AP-1/SMAD-binding sites [UniProt]. FOS plays critical roles in multiple physiological processes. In the central nervous system, FOS expression is rapidly and transiently induced by seizures, noxious stimuli, and cortical injury, making it a marker of brain metabolic activity 2. In skeletal tissue, FOS regulates chondrocyte proliferation and cellular bioenergetics by balancing pyruvate flux between anaerobic glycolysis and the tricarboxylic acid cycle—critical for cartilage integrity 1. In reproductive tissues, hCG-induced FOS/AP-1 activation regulates metabolic changes and cholesterol biosynthesis in granulosa cells 3, while FTO-mediated m6A demethylation of FOS-mRNA influences ovarian aging 4. Clinically, FOS dysregulation contributes to disease pathogenesis. FOS and FOSB gene rearrangements are associated with various tumors including epithelioid hemangioma and osteoid osteoma 5. In aldosterone-producing adenomas, FOS and JUN activation links oxidative stress to aberrant steroidogenesis 6, highlighting FOS's role in endocrine disorders.