Osteoblasts are derived from mesenchymal stem cells, common progenitors for adipocytes and other lineages. While transcription factors such as Runx2/Cbfa1 and Osterix are absolutely required for osteoblast differentiation, simply increasing their expression does not necessarily lead to increased bone formation. In contrast, genes such as deltafosB and an AP-1 target, interleukin-11, which have been shown to enhance bone formation in vivo and to be induced by mechanical stress and PTH, can be better therapeutic targets to develop drugs that stimulate bone formation.