CD3e (CD3-epsilon), together with CD3-gamma, -delta and -zeta, and the T-cell receptor alpha/beta and gamma/delta heterodimers, forms the T-cell receptor-CD3 complex. This complex plays an important role in coupling antigen recognition to several intracellular signal-transduction pathways. The gene for CD3e have been mapped to band q23 of chromosome 11 in humans and consisted of nine exons. Three exons, encoding the junction of leader peptide and mature protein, are extremely small. The CD3e cytoplasmic tail, highly conserved, can be tentatively subdivided into three regions; the N-terminal region contains a basic amino acid cluster, the central region contains a proline-rich sequence, and the C-terminal region contains the ITAM. Among the CD3 subunits, CD3e is the most likely candidate for performing a critical role in the TCR signaling response. CD3e is represented twice in the TCR complex, serving as a component of both the gamma/epsilon and delta/epsilon dimers. In addition, after TCR cross-linking, CD3e and zeta are the predominant tyrosine-phosphorylated TCR subunits.