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OX-2 membrane glycoprotein, also named CD200 （Cluster of Differentiation 200） is a human protein encoded by the CD200 gene. CD 200 belongs to the immunoglobulin superfamily, particularly belongs to the B7 receptor family. CD200 is overexpressed in cancer cells in a number of human tumors including melanoma, ovarian cancer, some B-cell malignances and small cell lung carcinoma. In the tumor microenvironment CD200 is also expressed in endothelial cells and activated T lymphocytes, B lymhocytes and myeloid cells. These cells can thus interact with cells expressing CD200R such as T regulatory cells, tumor-associated dendritic cells, tumor associated macrophages and myeloid derived suppressor cells （MDSC）. It was shown that CD200 expressed on tumor cells promotes expansion of MDSCs that are capable of inhibiting anti-tumor immune response. CD200 blockade inhibits tumor growth and decreases number of MDSCs in tumor tissue.