Cluster of differentiation (CD) molecules are cell surface markers widely used in flow cytometry for the identification and characterization of leukocytes and their subsets. These markers, which behave in a variety of sorts, typically act as receptors, co-receptors, ligands or cell adhesion molecules that mediate various immune responses. Since cells often express multiple CD markers at any given time, utilizing a combination of CD markers is especially useful in determining a subpopulation.