Insulin-like growth factor-binding protein 3, also known as IGFBP-3, is a protein that in humans is encoded by the IGFBP3 gene. IGFBP-3 is one of six IGF binding proteins. IGFBP-3 was first isolated, characterized, and quantitated in human plasma, in 1986. It has well-documented functions in the circulation, in the extracellular environment, and inside cells. It is the main IGF transport protein in the bloodstream, where it carries the growth factors predominantly in stable complexes that contain the binding protein, either IGF-1 or IGF-2, and a third protein called the acid-labile subunit or ALS. IGFBP-3 levels in human serum are, like IGF-1, dependent on growth hormone (GH); for example, serum IGFBP-3 is increased in acromegaly and low in GH-deficient children. However, IGFBP-3 gene expression in human liver is GH-independent. Because it is stabilized in human serum by forming complexes with IGF-1 and ALS, which are both GH-dependent, serum IGFBP-3 also appears regulated by GH. Its production by some non-hepatic tissues may also be directly GH-regulated. Immunoassays for serum IGFBP-3 are often used as part of the diagnosis of childhood GH-deficiency.