The calcitonin receptor is the best marker for distinguishing mammalian osteoclasts because this receptor is not expressed on monocyte/macrophage-derived giant cells. A wide variety of factors that influence osteoclast formation and functionincludesystemichormones,cytokines,andgrowthfactors.Itisnoteworthythat neither IL-4 (FBGCformation)nor°-interferon(Langhansgiantcellformation)is described as a significant factor in the formation or activation of osteoclasts. These f indings suggest that although the CFU-GMprogenitoris ofmonocyticlineage, its differentiation does not include expression of IL-4 or IFN-° receptors or, perhaps, even a common signal transduction pathway. This is somewhat surprising as both foreign body giant cells and osteoclasts adhere to substrates through podosomal structures