Costimulatory molecule B7-H4, a member of the B7 family, negatively regulates the immune response of T cells through inhibiting their proliferation, cytokine production and cell cycle. The overexpression of B7-H4 plays an important role in initiation, progression and regression of tumors by promoting malignant transformation of epithelial cells and protecting epithelial cells from anoikis. B7-H4 expression in malignant tumors is useful for clinical diagnosis, and may provide a novel molecular target for cancer treatment.