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. Function : IGF-binding proteins prolong the half-life of the IGFs and have been shown to either inhibit or stimulate the growth promoting effects of the IGFs on cell culture. They alter the interaction of IGFs with their cell surface receptors.