CD244, known as 2B4, is a 38 kD type I transmembrane protein. It is a member of the CD2 subset of the immunoglobulin superfamily (IgSF) molecules. CD244 is expressed on NK cells, a subset of T cells (including majority of CD8+ T cells and gamma/delta T cells), monocytes, basophils, and eosinophils. CD48 is the ligand of CD244. It has been reported that ligation of human CD244 results in enhanced NK cell cytotoxicity and cytokine production. Recent studies have shown that human CD244, like murine CD244, has both activating and inhibitory functions, which are dependent on the density of surface 2B4 expression, degree of ligation, and the level of the adaptor molecule SAP expression.