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-BP1 is a 25.4 kDa cysteine-rich secreted protein expressed in liver, deciduas, and kidneys and is the most abundant IGF-BP in amniotic fluid. Levels of IGF-BP1 in serum are lowest after food. IGF-BP1 binds to both IGF-I and IGF-II with equal affinity. Phosphorylated IGF-BP1 hinders IGF actions, where as nonphosphorylated IGF-BP1 is stimulatory. Recombinant human IGF-BP1 is a 25.4 kDa protein consisting of 235 amino acid residues (Isoform A).