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Any of a group of biologically active poly-peptides which function as hormonelike regulatory signals, controlling the growth and differentiation of responsive cells. Indeed, the distinction between growth factors and hormones is frequently arbitrary and stems more from the manner of their discovery than from a clear difference in function. See Cell differentiation, Hormone
The sequence of amino acids has been determined for several growth-factor polypeptides. This information permits a number of growth factors to be placed into families, members of which have related amino acid sequences, suggesting that they evolved from a single ancestral protein. The insulin family comprises somatemedins A and C, insulin, insulinlike growth factor (IGF), and multiplication-stimulating factor (MSF). A second family consists of sarcoma growth factor (SGF), transforming growth factors (TGFs), and epidermal growth factor (EGF). In addition, there are growth factors, such as nerve growth factor (NGF), fibroblast growth factor (FGF), and platelet-derived growth factor (PDGF), for which structural homologs have not been identified. See Insulin, Protein
The stimulation of cell proliferation by several growth factors is similar in some ways to the rapid cell proliferation characteristic of tumor cells. Furthermore, the growth factor receptors are similar to the tumor-causing proteins produced by several RNA tumor viruses. It has been demonstrated that platelet-derived growth factor is virtually identical to the tumor-causing protein of the RNA tumor virus, simian sarcoma virus. Some forms of cancer involve improper function of growth factors.