Background: Mouse models that include spontaneous mutations, as well as those carrying allelic differences, have been extremely useful in clarifying the role of skeletal and circulating insulin-like growth factor I (IGF-I) in peak bone acquisition. The role of peroxisome proliferator-activated receptor gamma, a nuclear receptor and key differentiation factor for adipogenesis, has provided new insights into the relationship of marrow fat to skeletal mass. Conclusions: Mouse models continue to deepen our understanding of IGR-I signaling and the role of IGR-I in determining marrow stromal cell fate. Copyright (c) 2007 S. Karger AG, Basel.