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IGF-I is a well-characterized basic peptide secreted by the liver that circulates in the blood. It has growth-regulating, insulin-like, mitogenic activities. IGF-I is a growth factor that has a major, but not absolute, dependence on somatotropin. It is believed to be mainly active in adults in contrast to IGF-II, which is also a major fetal growth factor. Human Long R3 Insulin-like Growth Factor-I （rhLR3-IGF-I） contains an 83 amino acid analog of human IGF-I. Compared to the complete human IGF-I sequence, an addition of the rhLR3-IGF-1 includes the substitution of an Arg for the Glu at position 3 （hence R3） and a13 amino acid extension peptide at the N-terminus. An enhanced potency is due to the markedly decreased binding of human Long R3-IGF-I to IGF binding proteins which normally inhibit the biological actions of IGFs.