IGFBP1, also known as IGFBP-1 and insulin-like growth factor-binding protein 1, is a member of the insulin-like growth factor-binding protein family. IGF binding proteins (IGFBPs) are proteins of 24 to 45 kDa. All six IGFBPs share 50% homology and have binding affinities for IGF-I and IGF-II at the same order of magnitude as the ligands have for the IGF-IR. 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. IGFBP1 has an IGFBP domain and a thyroglobulin type-I domain. It binds both insulin-like growth factors (IGFs) I and II and circulates in the plasma. The binding of this protein prolongs the half-life of the IGFs and alters their interaction with cell surface receptors.