CD3 is a multi-subunit complex consisting of 4 different invariable membrane proteins, CD3E, CD3D, CD3G and CD3-omega. These CD3 proteins are expressed on the surface of T-cells and Thymocytes during thymocyte development. They are non-covalently associated with the TCR heterodimer and form the functional TCR-CD3 antigen receptor complex of the T-lymphocyte. CD3 expression is significant during Intrathymic T-lymphocyte maturation, TCR signaling and Thymocyte differentiation. Stimulation of TCR-CD3 complex results in activation of various protein tyrosine kinases including ZAP-70 affecting multiple cellular responses including T-cell proliferation, clonal expansion, induction of tolerance and apoptosis. Mutations in the sub-unit chains of CD3 have been associated with various immunodeficiency syndromes including SCID.