Family of conserved peptide hormones structurally homologous with insulin. Two major circulating forms mediate the growth promoting effects of somatotropin, q.v. IGF-I regulates both prenatal and postnatal growth; IGF-II is a key factor in fetal development. IGFs are produced primarily in the liver under the regulation of growth hormone; also produced locally by most tissues. Transported in the serum by IGF binding proteins (or IGFBP) that prolong the half-life and regulate the metabolic effects of IGFs. Discovered and termed sulphation factors because of their ability to stimulate the incorporation of sulfate by cartilage: W. D. Salmon, W. H. Daughaday, J. Lab. Clin. Med. 49, 825 (1957) PubMed. These peptides have also been referred to as NSILA-S, or non-suppressible insulin-like acting substance. The designation somatomedin was proposed to connote the intermediary relationship to somatotropin: W. H. Daughaday et al., Nature 235, 107 (1972) DOI PubMed. Discussion of nomenclature: idem et al., J. Clin. Endocrinol. Metab. 65, 1075 (1987). Isoln, chemical characterization, biological properties of IGF-I and IGF-II: E. Rinderknecht, R. E. Humbel, Proc. Natl. Acad. Sci. USA 73, 2365, 4379 (1976) DOI PubMed. Amino acid sequence of IGF-I: eidem, J. Biol. Chem. 253, 2769 (1978) PubMed; of IGF-II: eidem, FEBS Lett. 89, 283 (1978) DOI PubMed. Total synthesis of human IGF-I: C. H. Li et al., Proc. Natl. Acad. Sci. USA 80, 2216 (1983) DOI PubMed; of human IGF-II: eidem, Biochem. Biophys. Res. Commun. 127, 420 (1985) DOI PubMed. Review of molecular biology: W. H. Daughaday, P. Rotwein, Endocr. Rev. 10, 68-91 (1989) DOI PubMed; of mechanism of action: A. Spagnoli, R. C. Rosenfeld, Endocrinol. Metab. Clin. North Am. 25, 615-631 (1996) DOI PubMed. Review of IGF binding proteins: D. R. Clemmons, Cytokine Growth Factor Rev. 8, 45-62 (1997) DOI PubMed. Reviews: C. E. H. Stewart, P. Rotwein, Physiol. Rev. 76, 1005-1026 (1996) PubMed; D. Le Roith, N. Engl. J. Med. 336, 633-640 (1997) DOI PubMed.