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). Expressed in thymus and spleen, with highest levels in immature thymocytes (at protein level). Phosphorylated on tyrosines upon TCR activation; which leads to the recruitment of PTPN11, GRB2 and CSK. Mice are viable and fertile, with normal growth and no obvious abnormalities. They show normal B-cell development and peripheral B-cell functions, but have mildly altered thymic development probably due to enhanced TCR signaling. This results in a greater susceptibility to autoimmune diseases.