CD99 is a glycoprotein found on the leukocytes surface. It has been variously described as a human thymus leukemia Ag (Levy et al. 1979), a Ewing's sarcoma-specific membrane marker molecule (Hamilton et al. 1988) and a putative adhesion molecule (termed E2) involved in spontaneous rosette formation of T cells with erythrocytes (Aubrit et al. 1989, Bernard et al. 1988). CD99L2 is a paralog of CD99 that directly interacts with CD99 to form a heterodimer via its cytoplasmic domain. This interaction positively regulates CD99L2 trafficking to cell surfaces (Nam et al. 2013).