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 spleen, thymus, and peripheral blood leukocytes. Expressed in several B-, T-, NK and monocyte cell lines. Up-regulated in T-cells following TCR engagement. Phosphorylated on tyrosines by Syk, Lck or ZAP70 upon TCR or BCR activation; which leads to the recruitment of GRB2, PIK3R1 and GRAP2.