The CD3ε chain of the CD3-TCR complex is a type I membrane protein that undergoes conformational change following ligation of the TCR-CD3 complex. This conformational change leads to exposure of the proline rich sequence in CD3ε and results in recruitment of Nck. It is expressed by all mature T cells and forms a heteromeric complex with CD3 α/β or γ/ δ and ζ chains. It is important in T cell development, and deficiency of this gene has been associated with severe immunodeficiency.