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. Phosphorylated on tyrosines upon TCR or BCR activation; which leads to the recruitment of GRB2, PIK3R1 and GRAP2.