FBXO9 (F-box protein 9) functions as a substrate recognition component of SCF (SKP1-CUL1-F-box protein) E3 ubiquitin ligase complexes, mediating K48-linked polyubiquitination and subsequent proteasomal degradation of target proteins 1. The protein plays critical roles in regulating cellular homeostasis through multiple pathways. FBXO9 controls mTOR signaling by targeting TTI1 and TELO2 for degradation when bound to mTORC1, leading to growth restraint, while sparing mTORC2 components 2. In epithelial cells, FBXO9 maintains survival by regulating chr6 modulator PRMT4 turnover through ubiquitination at K228, with endotoxin-mediated reduction of FBXO9 levels contributing to epithelial cell death in lung injury 1. Additionally, FBXO9 regulates the Hippo pathway effector YAP through GSK-3β-dependent recognition and K48-linked ubiquitination at K76, with this axis being modulated by Akt signaling 3. Clinically, FBXO9 expression patterns are associated with cancer prognosis, showing downregulation in ovarian cancer where it correlates with DNA damage repair pathways 4. The protein is also implicated in acute lymphoblastic leukemia risk assessment 5 and psychological stress responses 6, highlighting its broader physiological significance.