Adapter protein, which negatively regulates T-cell receptor (TCR) signaling. Inhibits T-cell antigen-receptor induced activation of nuclear factor of activated T-cells. May act by linking signaling proteins such as ZAP70 with CBL, leading to a CBL dependent degradation of signaling proteins. Interacts (via its C-terminal domain) with CBL (phosphorylated). Interacts (via SH2 domain) with ZAP70 (phosphorylated) and CD3Z (phosphorylated). Interacts (via SH2 domain) with CSF1R (phosphorylated). Localized to the plasma membrane and intracellular vesicles, including late endosomal vesicles. Mainly expressed in immune system. Highly expressed in spleen and thymus and expressed at intermediate levels in lung. Not expressed in liver, heart and brain. Isoform 1 is predominant in lung and spleen, while isoform 2 is predominant in thymus. The loss of the C-terminal domain partially abolishes the inhibitory function. Phosphorylated by CSF1R. PubMed:12024036 confirmed the alternative initiation by mutating the Met in position 1 to Val, and showed that isoform 1 is abolished in favor of isoform 2.