Tax⁺OPG^Tg mice are protected from osteolytic bone lesions and soft tissue tumor development. Tax⁺ mice were crossed with OPG^Tg mice. Tax⁺OPG^Tg mice (n = 33) and Tax⁺OPG^WT (n = 22) littermates were evaluated weekly for peripheral tumor formation, and radiographic images were taken at 3, 6, and 9 months of age. Experiments were ended when mice were 9 months old. (A) Representative radiographic images of a Tax⁺OPG^Tg mouse and a Tax⁺OPG^WT mouse at 9 months of age. Tax⁺OPG^WT mouse demonstrates osteolytic bone lesions on the tail vertebrae (arrows). (B) In mice that developed osteolysis, Tax⁺OPG^WT mice show significantly increased bone lesions on the tail vertebrae (P < .01) compared with Tax⁺OPG^Tg. Bone lesion number is depicted as the mean ± SEM. (C) Tax⁺OPG^Tg mice are protected from the development of soft-tissue tumors (P < .01, by a paired t test in SigmaPlot 2001 version 9; Systat Software, Point Richmond, CA).