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 SH2 domain) with ZAP70 (phosphorylated) and CD3Z (phosphorylated). Interacts (via SH2 domain) with CSF1R (phosphorylated) (By similarity). Interacts (via its C-terminal domain) with CBL (phosphorylated). Localized to the plasma membrane and intracellular vesicles, including late endosomal vesicles. May be cytoplasmic and is not localized to membranes. Predominantly expressed in immune system, with highest levels in peripheral blood leukocytes. Expressed in spleen, thymus and lymph nodes. Expressed in T-cells as well as in monocytes, and at low level in B-cells. Also detected in placenta, prostate, skin, retina and colon. The loss of the C-terminal domain partially abolishes the inhibitory function, but can be partially compensated by higher level of protein expression. Phosphorylated by CSF1R. Produced by alternative initiation at Met-28 of isoform 1. Produced by alternative initiation at Met-28 of isoform 2.