Lck and TCR complex. a) The hierarchical substructure of protein tyrosine kinase Lck. There are three domains in Lck: a Src homology 3 (SH3) domain, a Src homology 2 (SH2) domain, and a protein tyrosine kinase (PTK) domain. Three tyrosine residues are subject to phosphorylation and dephosphorylation (Y192, Y394 and Y505). b) The structure of the TCR complex. The TCR complex is composed of αβ, γϵ, and δϵ heterodimers and a ζζ homodimer. Black boxes represent ITAMs.