C1QBP (complement C1q binding protein) is a multifunctional protein with roles spanning immune regulation, DNA damage response, and cancer biology. Primary function: C1QBP binds complement C1q and serves as a cell surface and cytosolic receptor 1. Mechanism: The protein operates through multiple pathways—it stabilizes the MRE11/RAD50/NBS1 (MRN) complex to regulate homologous recombination and DNA repair 2, and serves as a receptor for vitronectin (Vtn) on macrophage surfaces, where Vtn-C1QBP interaction suppresses macrophage phagocytosis 3. Additionally, fibroblast-derived C1q acts on T cell C1QBP to prevent apoptosis and promote CD4 T cell immunity 4. Disease relevance: C1QBP is implicated in cancer progression through multiple mechanisms: exosomal CD44v6/C1QBP complexes promote pancreatic cancer liver metastasis by activating hepatic stellate cells 5, circRNA-mediated C1QBP stabilization drives laryngeal squamous cell carcinoma through Wnt/β-catenin activation 6, and C1QBP expression correlates with AML relapse risk 7. Clinical significance: C1QBP represents a therapeutic target—blocking Vtn-C1QBP interaction sensitizes tumors to immunotherapy 3, and C1QBP inhibition enhances chemotherapy efficacy in breast cancer 2. Elevated plasma C1QBP peptides associate with Alzheimer's disease 8.