Initial identification of osteoactivin (OA)/glycoprotein non-melanoma clone B (gpnmb) was demonstrated in an osteopetrotic rat model, where OA expression was increased 3-fold in mutant bones, compared to normal. OA mRNA and protein expression increase during active bone regeneration post-fracture, and primary rat osteoblasts show increased OA expression during differentiation in vitro. To further examine OA/gpnmb as an osteoinductive agent, we characterized the skeletal phenotype of transgenic mouse overexpressing OA/gpnmb under the CMV-promoter (OA-Tg).