The Role of Semaphorin 4D (Sema4D) in Bone Metastasis Background: Bone metastasis is a catastrophic endpoint of many neoplastic diseases, but especially for patients with advanced breast cancer. Despite the continuous advances in pharmacological and cancer research, bone loss and subsequent bone complications are seen in 70% of females diagnosed with breast cancer. Semaphorin 4D (Sema4D), a protein originally described to regulate the immune response, is now known to have a novel role in bone regulation. Sema4D is also found to be highly expressed by many tumor cells including those of breast cancer. In this study we focus on the role of Sema4D produced by tumor cells on their ability to metastasize to bone. Materials and methods: The osteoblast cell line MC3T3 was treated under different osteogenic conditions to examine the effects of Sema4D on bone differentiation in vitro. We also used tumor cells with silenced Sema4D to investigate the effects of tumor-derived Sema4D on their ability to metastasize to bone in vivo. Results: Sema4D produced by the breast cancer cell line MDA-231 inhibited bone matrix formation and mineralization in vitro. In vivo, however, MDA-231 tend to spread to bone only when Sema4D was highly expressed by these cells and not when it was silenced. Conclusion: Over-expression of Sema4D by breast cancer cells inhibits bone formation in vitro and tends to increase the ability of these cells to metastasize to bone in vivo and establish osteolytic lesions characterized by this tumor type. Our findings may serve as a solid starting point to investigate the role of anti-Sema4D therapy in tumor metastasis. Further in vivo studies are strongly encouraged to clinically determine their effects.