Given that high levels of Dkk-1 are also expressed by rapidly dividing osteosarcoma (OS) cells in vitro (Gregory et al, 2003, 2005a, 2005b), we measured the level of Dkk-1 in the serum of paediatric patients with OS (median age, 13.4 years) and found it to be significantly elevated when compared with healthy controls. The gene discussed is DKK1; the disease is osteosarcoma.