RANKL (Receptor Activator of Nuclear Factor Kappa-B Ligand), a key cytokine belonging to the tumor necrosis factor (TNF) family. RANKL is produced by osteoblasts and other cells (e.g., activated T cells). It binds to its receptor RANK on the surface of osteoclast precursors and mature osteoclasts, promoting their differentiation, activation, and survival, thereby driving bone resorption. Denosumab acts as a high-affinity RANKL inhibitor.