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. Promotes cell migration. Binds equally well IGF1 and IGF2. Phosphorylated; probably by casein kinase II. Phosphorylation alters the affinity of the protein for IGFs. In amniotic fluid, the unmodified protein is the most abundant form, while mono-, bi-, tri- and tetraphosphorylated forms are present in decreasing amounts. The phosphorylation state may influence the propensity to proteolysis.