B7-H4, also known as B7x and B7S1, is an immune costimulatory protein that is expressed on the surface of activated lymphocytes, macrophages, monocytes, dendritic cells, epithelial cells, and bone marrow-derived mesenchymal stem cells. Its binding to activated T cells dampens T cell responses and induces cell cycle arrest in the T cell. Reverse signaling can induce either cell cycle arrest or apoptosis in the B7-H4 expressing cell. B7-H4 is upregulated in several carcinomas in correlation with tumor progression and metastasis. A soluble form of B7-H4 functions as a decoy molecule and is elevated in the serum of ovarian cancer, renal cell carcinoma, and rheumatoid arthritis patients.