Negatively regulates T-cell antigen receptor (TCR)-mediated signaling. Involved in positive selection of T-cells. Homodimer; disulfide-linked. When phosphorylated, interacts with PTPN11/SHP2, GRB2 and CSK (By similarity). Lymph node, spleen and thymus. Phosphorylated on tyrosines upon TCR activation; which promotes recruitment of PTPN11, GRB2 and CSK.