The cluster of differentiation (CD) system is commonly used as cell markers in immunophynotyping. Different kinds of cells in the immune system can be identified through the surface CD molecules which associating with the immune function of the cell. There are more than 320 CD unique clusters and subclusters have been identified. Some of the CD molecules serve as receptors or ligands important to the cell through initiating a signal cascade which then alter the behavior of the cell. Some CD proteins do not take part in cell signal process but have other functions such as cell adhesion. Cell surface glycoprotein CD200 receptor 1 (CD200R1) is an isoform of CD200 receptors which is expressed on cells of the myeloid lineage. CD200R1 is a receptor for the OX-2 membrane glycoprotein. The receptor-substrate interaction may serve as a myeloid downregulatory signal.