IGFBPs control the distribution, function and activity of IGFs in various cell tissues and body fluids. Currently, there are seven named IGFBPs that form high affinity complexes with both IGF-I and IGF-II. IGFBP-5 is a 28.6 kDa, cysteine-rich, secreted protein produced by vascular smooth muscle cells. It is the major IGF-binding protein present in bone tissue and helps potentiate the action of IGF-I on smooth muscle cells, fibroblasts, and osteoblasts. Data shows that IGFBP-5 acts as a growth inhibitor and pro-apoptotic agent in breast cancer cells. IGFBP-5-overexpressing mice show an increase in neonatal mortality, reduced female fertility, whole-body growth inhibition, and retarded muscle development.