Osteoblasts and adipocytes may develop from common bone marrow mesenchymal precursors. Transgenic mice overexpressing ΔFosB, an AP-1 transcription factor, under the control of the neuron-specific enolase (NSE) promoter show both markedly increased bone formation and decreased adipogenesis. To determine whether the two phenotypes were linked, we targeted overexpression of ΔFosB in mice to the osteoblast by using the osteocalcin (OG2) promoter. OG2-ΔFosB mice demonstrated increased osteoblast numbers and an osteosclerotic phenotype but normal adipocyte differentiation. This result firmly establishes that the skeletal phenotype is cell autonomous to the osteoblast lineage and independent of adipocyte formation. It also strongly suggests that the decreased fat phenotype of NSE-ΔFosB mice is independent of the changes in the osteoblast lineage. In vitro, overexpression of ΔFosB in the preadipocytic 3T3-L1 cell line had little effect on adipocyte differentiation, whereas it prevented the induction of adipogenic transcription factors in the multipotential stromal cell line ST2. Also, ΔFosB isoforms bound to and altered the DNA-binding capacity of C/EBPβ. Thus, the inhibitory effect of ΔFosB on adipocyte differentiation appears to occur at early stages of stem cell commitment, affecting C/EBPβ functions. It is concluded that the changes in osteoblast and adipocyte differentiation in ΔFosB transgenic mice result from independent cell-autonomous mechanisms.