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 5% homology with each other 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. Binding of this protein prolongs the half-life of the IGFs and alters their interaction with cell surface receptors.