Human IGF-BP5 is a 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. Studies show that IGF-BP5 acts as a growth inhibitor and pro-apoptotic agent in breast cancer cells. IGF-BP5 overexpressing mice show an increase in neonatal mortality, reduced female fertility, whole body growth inhibition, and retarded muscle development.