Immune costimulatory protein B7-H4 (B7-H4) is expressed on the surface of a variety of immune cells and functions as a negative regulator of T cell response. By arresting cell cycle, B7-H4 inhibits growth, cytokine secretion and cytotoxicity development. When expressed on the cell surface of tumor macrophages, B7-H4 plays an important role, together with regulatory T-cells (Treg), in the suppression of tumor-associated antigen-specific T-cell immunity. B7-H4 is also involved in promoting epithelial cell transformation. B7-H4 can be a predictive marker for renal cell carcinoma.