In light of the reports that DKK-1 can be measured in the serum of osteosarcoma patients, that DKK-1 can inhibit osteoblastic differentiation, and that overexpression of DKK-1 appears to induce a more aggressive phenotype in osteosarcoma cells implanted in an orthotopic location, we investigated DKK-1 expression in an orthotopic patient-derived xenograft model of osteosarcoma as well as the effect of anti-DKK-1 antibodies on growth and metastasis of patient-derived osteosarcoma xenografts. This evidence concerns the gene DKK1 and osteosarcoma.