Exploration of anabolic pathways in osteoblasts revealed that Zfp521, a 30-zinc finger protein, is highly expressed at the periphery of mesenchymal condensations and in developing bones. In these structures it is expressed in chondroblasts, prehypertrophic chondrocytes, the periosteum, osteoblasts, osteoblast precursors, and osteocytes. Forced expression of Zfp521 in osteoblasts in vivo increases bone formation and bone mass, whereas preliminary data suggest that germline deletion leads to osteopenia. In contrast, overexpressing Zfp521 in vitro antagonizes, and knockdown favors, osteoblast differentiation and nodule formation. Zfp521 expression is inhibited by bone morphogenetic protein-2 and stimulated by parathyroid hormone-related protein. Mechanistically, Zfp521 binds to Runx2, repressing its transcriptional activity. These data support the hypothesis that Zfp521 both opposes the progression of precursors and promotes the maturation and function of mature osteoblasts. The balance between Zfp521 and Runx2 may therefore contribute to the regulation of osteoblast differentiation and bone formation.