Negatively regulates TCR (T-cell antigen receptor)-mediated signaling in T-cells. Involved in positive selection of T-cells. Homodimer; disulfide-linked. When phosphorylated, interacts with PTPN11/SHP2, GRB2 and CSK. Specifically expressed in T- and B-cells. Present in plasma cells but not in germinal center B-cells (at protein level). Expressed in T- and B-cell lymphoma. Phosphorylated on tyrosines by LCK, FYN or ZAP70 upon TCR activation; which leads to the recruitment of PTPN11, GRB2 and CSK. In contrast to its orthologs it harbors a signal sequence.