Figure 6: T cell receptor (TCR) signal transduction is initiated by the recognition of cognate peptide-MHC molecules. The first molecule to be recruited to the TCR-CD3 complex is the SRC family kinase (SFK) member LCK, which phosphorylates immunoreceptor tyrosine-based activation motifs (ITAMs) of the CD3γ chain, CD3δ chain, CD3ε chains, and the ζ-chains.