Stabilizes the TCR (T-cell antigen receptor)/CD3 complex at the surface of T-cells. Homodimer; disulfide-linked. Interacts with CD3Z. When phosphorylated, interacts with PIK3R1. Strongly expressed in thymus, and to a lesser extent in spleen, lymph node and peripheral blood lymphocytes. Present in T-cells and NK cells, but not B-cells (at protein level). Strongly expressed in fetal thymus at weeks 17-24 of gestation. Undetectable in bone marrow and fetal liver. Phosphorylated on tyrosines by LCK or FYN upon TCR activation.