BACKGROUND The Syk family protein tyrosine kinase Zap70 is expressed in T and NK cells and plays a critical role in mediating T cell activation in response to T cell receptor (TCR) engagement (1). Following TCR engagement, Zap70 is rapidly phosphorylated on several tyrosine residues through autophosphorylation and transphosphorylation by the Src family tyrosine kinase Lck (2-6). Tyrosine phosphorylation correlates with increased Zap70 kinase activity and downstream signaling events. Expression of Zap70 is correlated with disease progression and survival in patients with chronic lymphocytic leukemia (CLL) (7, 8).
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