Negatively regulates TCR (T-cell antigen receptor)-mediated signaling in T-cells and BCR (B-cell antigen receptor)-mediated signaling in B-cells. When phosphorylated, interacts with GRB2, PIK3R1 and GRAP2. Expressed in T-cells and B-cells. Up-regulated in T- and B-cells following TCR and BCR engagement, respectively. Phosphorylated on tyrosines upon TCR or BCR activation; which leads to the recruitment of GRB2, PIK3R1 and GRAP2. Mice are viable, fertile and healthy, and show normal lymphocyte development. However, their T- and B-cells are hyperresponsive to stimulation via TCR or BCR.