TCR stimulation induce the transient dephosphorylation of PAG thereby release the Csk from its plasma membrane anchor. The release of Csk from its proximity with Lck may serve to facilitate the activation of Lck.Protein tyrosine phosphtase CD45 (PTPRC) and CD148 (PTPRJ) have dual function in TCR signalling. They act both in activation as well as inactivation of Src family kinases (SFKs) which are involved in the initiation of TCR signal transduction (Stepanek et al. 2011). The activatory role is to dephosphorylate an inhibitory site tyrosine 505 (Y505) at the C-terminal end of Lck, which is needed to enable Lck to an open conformation and expose the activation loop (A-loop) containing the activating tyrosine 394 (Y394) (Xu et al. 1993. McNeill et al. 2007, Zikherman et al. 2010, Stepanek et al. 2011, Salmond et al. 2009).