T-Cell Surface Glycoprotein CD3 ε Chain (CD3ε) is a single-pass type I membrane protein. CD3ε contains 1 Ig-like (immunoglobulin-like) domain and 1 ITAM domain. CD3ε is a polypeptide encoded by the CD3E gene on chromosome 11 in humans. The T cell receptor-CD3 complex (TCR/CD3 complex) is involved in T-cell development and several intracellular signal-transduction pathways. This complex is critical for T-cell development and function, and represents one of the most complex transmembrane receptors. The T cell receptor-CD3 complex is unique in having ten cytoplasmic immunoreceptor tyrosine-based activation motifs (ITAMs). TCR/CD3 complex plays an important role in coupling antigen recognition to several intracellular signal-transduction pathways.