LAT is an integral membrane protein which is phosphorylated by ZAP-70/Syk protein tyrosine kinases following activation of the T-cell antigen receptor (TCR) signal transduction pathway. LAT is expressed in T lymphocytes in interstitial spaces, platelets and megakaryocytes. LAT is not expressed in B cells, macrophages, plasma cells, monocytes, epithelial histiocytes and dendritic cells.