Serum levels of RANKL were significantly higher in patients with sickle cell disease than in controls (1.18±0.72 pmol/l vs. 0.29±0.26 pmol/l, P = 0.0002), whereas levels of osteoprotegerin (OPG) were not altered in these patients. The gene discussed is TNFSF11; the disease is sickle cell disease.