Media from syndecan-1–expressing cells inhibit osteoclast formation in coculture assay. Marrow cells were cocultured with calvaria cells for 8 days in the absence (control) or presence of 10 nmol/L 1,25(OH)₂D₃, or 1 μL of conditioned media from vector-transfected (neo) or syndecan-1–transfected (syn-1) myeloma cells. The number of osteoclasts was determined by counting TRAP-positive cells with greater than 3 nuclei per cell (*P < .05).