Involved in BCR (B-cell antigen receptor)-mediated signaling in B-cells and TCR (T-cell antigen receptor)-mediated T-cell signaling in T-cells. In absence of TCR signaling, may be involved in CD4-mediated inhibition of T-cell activation. Couples activation of these receptors and their associated kinases with distal intracellular events such as calcium mobilization or MAPK activation through the recruitment of PLCG2, GRB2, GRAP2, and other signaling molecules. When phosphorylated in response to TCR stimulation and/or CD4 costimulation, interacts with LCK, CSK, FYN, PTPN11/SHP2, GRB2, PIK3R1 and GRAP2 (By similarity). When phosphorylated in response to BCR activation, interacts with LYN, PIK3R1, PLCG2 and GRB2. Present in lipid rafts. Recruited to the immunological synapse upon conjugation of T-cell with antigen-presenting cell. Expressed in spleen and lung. Present in primary B-cells and peripheral T-cells (at protein level). Up-regulated in T-cells following TCR engagement. Palmitoylation of Cys-28 and Cys-31 is required for raft targeting. Phosphorylated on tyrosines upon TCR activation and/or CD4 coreceptor stimulation, or upon BCR stimulation; which leads to the recruitment of SH2-containing proteins.