 The intracellular tails of the CD3γ, CD3ε, and CD3δ molecules each contain a single conserved motif known as an "immunoreceptor tyrosine-based activation motif" or ITAM for short, which is essential for the signaling capacity of the TCR.The intracellular tail of CD3ζ contains 3 ITAM motifs.Phosphorylation of the ITAM on CD3 renders the CD3 chain capable of binding an enzyme called ZAP70 (zeta associated protein), a kinase that is important in the signaling cascade of the T cell.