CD3 epsilon chain <p>The CD3 epsilon protein is one of the invariant signalling subunits that are associated with the antigen-specific T cell receptor (TCR) alpha and beta chains. CD3 epsilon consists of a short extracellular segment, a transmembrane domain, and a cytoplasmic sequence containing a single immunoreceptor tyrosine-based activation motif (ITAM). Ligation of the variable subunits of the TCR triggers phosphorylation of tyrosines in the CD3 epsilon ITAM by tyrosine kinases such as Lck and Fyn, providing binding sites for a number of downstream effector molecules [<cite idref="PUB00016636"/>]. CD3 epsilon might play a unique role in T cell survival [<cite idref="PUB00034819"/>].</p>