Many tyrosine kinases, such as Lck, c-Src, and Itk, can activate themselves through trans-autophosphorylation of their activation loops (bottom panel, left side). These kinases can efficiently phosphorylate typical tyrosine kinase substrates, which have a near-neutral net charge (bottom panel, right side). ZAP-70 activation requires phosphorylation of its activation loop by Lck (top left panel). Once activated, ZAP-70 exclusively phosphorylates LAT and SLP-76, but not typical tyrosine kinase substrates, and LAT and SLP-76 cannot be phosphorylated by kinases like Lck, c-Src, and Itk (top right panel). Thus, ZAP-70 and its substrates are insulated from other proteins in the T cell receptor signaling pathway.