The ligand for osteoprotegerin (OPGL) directly activates mature osteoclasts - Burgess, T; Qian, Y X; Kaufman, S; Ring, B D; Van, G; Capparelli, C; Kelley, M; Hsu, H; Boyle, W J; Dunstan, C R; Hu, S; Lacey, D L (J Cell Biol, 1999)