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 (By similarity). Present in T-cells (at protein level). Present in thymocytes from 16.5 dpc (at protein level). Phosphorylated on tyrosines upon TCR activation. Strongly expressed at 13.5 dpc, then down-regulated. Marginal in adult. Protein may be unstable.