IGF-BPs controls the distribution, function and activity of IGFs in various cell tissues and body fluids. Currently there are seven named IGF-BPs that form high affinity complexes with both IGF-I and IGF-II. IGF-BP5 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 or 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. Recombinant human IGF-BP5 is a 28.6 kDa protein consisting of 253 amino acid residues.