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 BCR activation, interacts with LYN, PIK3R1, PLCG2, and GRB2 (By similarity). When phosphorylated in response to TCR stimulation and/or CD4 co-stimulation, interacts with LCK, CSK, FYN, PTPN11/SHP2, GRB2, PIK3R1 and GRAP2. Present in lipid rafts. Recruited to the immunological synapse upon conjugation of T-cell with antigen-presenting cell. Expressed in peripheral blood lymphocytes, lymphoid tissues, and liver. Present in T-cells and plasma cells, and in various hematopoietic cell lines (at protein level). 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.