IGFBP5 is one of a family of proteins that bind to insulin-like growth factors (IGFs). IGFBP5 is a multifunctional protein which acts not only as a traditional binding protein but also functions as a growth factor independent of IGFs to stimulate bone formation. Currently there are seven named IGF-BPs that form high affinity complexes with both IGFI and IGFII. IGFBP5 is the major IGF-binding protein present in bone tissue and helps potentiate the action of IGFI on smooth muscle cells, fibroblasts or osteoblasts. Data shows that IGFBP5 acts as a growth inhibitor and pro-apoptotic agent in breast cancer cells. IGFBP5 overexpressing mice show an increase in neonatal mortality, reduced female fertility, whole-body growth inhibition and retarded muscle development.