•In T-cells, Fyn and Lck carry out the initial phosphorylation.
•Lck is constitutively associated with the cytoplasmic region of either CD4 or CD8 while Fyn associates with the cytoplasmic domains of the CD3 complex and the ζ homodimer.
•Clustering following antigen recognition allows these Src kinases to phosphorylate the ITAMs on these chains
•Provides substrates for “second level” kinase, Zap70 (more later on this).