Involved in FCER1 (high affinity immunoglobulin epsilon receptor)-mediated signaling in mast cells. May also be involved in BCR (B-cell antigen receptor)-mediated signaling in B-cells and FCGR1 (high affinity immunoglobulin gamma Fc receptor I)-mediated signaling in myeloid cells. Couples activation of these receptors and their associated kinases with distal intracellular events through the recruitment of GRB2. When phosphorylated, interacts with GRB2. May also interact with SOS1, GAB1 and CBL. Present in lipid rafts. Strongly expressed in testis. Expressed in heart, spleen and lung. Present in B-cells and mast cells (at protein level). Hardly expressed in pro-B and pre-B cells. Moderately expressed in immature B-cells, mature B-cells and plasma cells. Highly expressed in transitional B-cells. Phosphorylated on tyrosines following cross-linking of BCR in B-cells, high affinity IgG receptor (FCGR1) in myeloid cells, or high affinity IgE receptor (FCER1) in mast cells; which induces the recruitment of GRB2. Mice exhibit normal T-cell, B-cell and mast cell development and normal humoral response, but have hyperresponsive mast cells.