Fig. 4. Proposed model of Sox4 regulation of osteoblast proliferation and differentiation. Represented is the hierarchy of the osteoblast lineage and the predicted Sox4 actions. According to the experimental data, Sox4 could affect proliferation of osteoprogenitors as well as differentiation of mature osteoblasts. This latter action could be exerted by downregulation of osterix, with a mechanism apparently independent of Runx2.