CD3 in conjunction with T-cell receptor (TCR) makes the TCR complex (Figure ). Surface membrane CD3 is a pan-T-cell marker and is expressed by thymocytes and all mature T-cells of peripheral blood and lymphoid tissues. CD3 is an essential T cell co-receptorand defines T cell lineage. CD3is therefore an ideal T cell marker. This mini-review explains the structure of CD3, the cell activation overview · CD3 protein structure · CD3 genes and function.