Receptor activator of nuclear factor kappa-B ligand (RANKL), also known as TNF-related activation-induced cytokine (TRANCE), is a cytokine that belongs to the tumor necrosis factor (TNF) superfamily. In humans, RANKL is primarily expressed by osteoblasts, T cells, and other immune cells, and plays a crucial role in regulating bone metabolism, immune function, and the development of mammary glands. RANKL functions by binding to its receptor RANK on the surface of osteoclast precursors, leading to the activation and differentiation of osteoclasts, which are responsible for bone resorption. Additionally, RANKL is involved in the regulation of T cell activation, dendritic cell function, and the formation of lactating mammary glands. Dysregulation of RANKL has been implicated in various bone-related diseases, such as osteoporosis and bone metastasis, as well as in immune disorders, highlighting the importance of RANKL in skeletal biology and immune regulation.