Insulin-like growth factor-1 (IGF1) has numerous biological roles. Its bioactivity is modulated by binding to IGF-binding proteins (IGFBPs) present in serum and tissues. Elis et al. studied the role of unbound (bioavailable) IGF1 in mouse strains expressing forms of IGF1 with low affinity for IGFBPs. They find that the mutant mice have increased body weight, length and relative lean mass, enlarged organs and enhanced mammary gland complexity. These data underscore the importance of tightly regulated IGF1 bioactivity for normal growth and development. Page 649

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