Pegvisomant, by virtue of its G120K mutation, prevents dimerization of the GHR and therefore all downstream signaling.(11,12) Pegvisomant has been well established as a clinical approach to normalize serum IGF-1 levels in patients with acromegaly,(13) and it is also known to normalize markers of bone formation and resorption.(14,15) By administering pegvisomant to LID mice, which have significantly reduced serum IGF-1 levels, the contribution of GH to skeletal development can be assessed independent of changes in hepatic IGF-1 production. This evidence concerns the gene GH1 and acromegaly.