Insulin-like growth factor binding protein 4 (IGFBP4) is a 24 kDa protein that binds insulin-like growth factors (IGFs) I and II and circulates in plasma in both glycosylated and non-glycosylated forms. This binding extends the half-life of the IGFs and alters their interaction with cell surface receptors. The superfamily of insulin-like growth factor (IGF)-binding proteins consists of six high-affinity IGF-binding proteins (IGFBPs) and at least four low-affinity binding proteins known as IGFBP-associated proteins (IGFBP-RPs.) IGFBP-4 inhibits the action of IGF-I in vitro, and it is the most abundant IGF-binding protein in the arterial wall of rodents. In nontransgenic littermates, IGFBP4 mRNA expression was highest in the liver and kidney.