B7-H4 is a protein that in humans is encoded by B7-H4 gene. B7-H4 belongs to the B7 family of costimulatory proteins. These proteins are expressed on the surface of antigen-presenting cells and interact with ligands (CD28) on T lymphocytes. It inhibits T cell proliferation and cytokine production through ligation of an unknown receptor expressed by activated T cells. Notably, B7-H4 protein expression is observed in a high proportion of patients' tumors across a wide variety of malignancies. This high expression by tumors in combination with its low or absent protein expression in normal tissues makes B7-H4 an attractive immunotherapeutic target.