Activation of T cells is dependent on T cell receptors (TCR) and CD28, which recognize the Major Histocompatibility Complex (MHC) peptide and B7 family members, respectively, on Antigen Presenting Cells (APC). T cell antigen recognition is initiated by the MHC presenting a short peptide sequence which is recognized by T cells expressing a specific TCR. This initiating event is followed by co-stimulation, primarily of CD28 on T cells by B7-1/CD80 and B7-2/CD86 proteins on the APC. Other proteins present in the TCR complex include CD3 proteins. Phosphorylation of immuno-receptor tyrosine-based activation motifs (ITAM), motifs on CD3ε by Lck promotes aggregation of signaling complexes and initiates further signal transduction cascades.