B7-H4, also known as VTCN1, is a type I transmembrane protein and member of the B7 family. Its extracellular region consists of one IgV-like and one IgC-like domain. B7-H4 expression has been reported on activated T cells, B cells, monocytes, and dendritic cells. On T cells, B7-H4 inhibits proliferation, cytokine secretion, and cytotoxicity. B7-H4 is also expressed by different carcinomas including renal, gastric, breast, ovarian and melanoma. Its expression is associated with a poor prognosis.