The closely related CD3 gamma and delta subunits constitute part of the invariant portion of the T cell receptor (TCR) complex. Both are transmembrane proteins with immunoglobulin-like domains in their extracellular regions. Each protein contains a single immunoreceptor tyrosine-based activation motif (ITAM) in its cytoplasmic domain that becomes phosphorylated upon ligation of the invariant TCR chains and provides binding sites for various downstream effectors [].
