Background: Painful breast carcinoma metastases in bone are a common manifestation of malignant disease. Eradication of these tumors can be evasive, and as a result, skeletal morbidity increases with disease progression. Experimental Design: The treatment potential of cytosine deaminase (CD) gene therapy combined with radiation treatment was evaluated in vitro and in vivo using a 4T1 murine breast carcinoma model. 4T1 carcinoma cells were transduced with a fusion gene encoding the extracellular and transmembrane domains of the human nerve growth factor receptor and the cytoplasmic portion of the yeast CD gene (NGFR-CDy). Results and Conclusions: CD-expressing tumor cells (4TCDy) were highly sensitive to treatment by 5-fluorocytosine prodrug (P < 0.0001). 5-Fluorocytosine treatment of 4TCDy, but not 4T1 cells, enhanced the effects of radiation in vitro (P < 0.0001). 5-Fluorocytosine prodrug treatment also increased the therapeutic potential of radiation in vivo. Mice with 4TCDy intrafemoral tumors showed increased effectiveness of radiation based on improved reductions in tumor size, reductions in tumorigenic osteolysis, and a decrease in skeletal fractures (P < 0.01).