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ZAP-70 （Zeta-chain-associated protein kinase 70） is a cytoplasmic protein tyrosine kinase that plays a crucial role in T cell receptor （TCR） signaling and is essential for T cell activation and development. It is primarily expressed in T cells, natural killer cells, and a subset of B cells. When the TCR binds to an antigen, the tyrosine kinase Lck is activated, phosphorylating the immunoreceptor tyrosine-based activation motifs （ITAMs） on the CD3 complex. These phosphorylated ITAMs serve as docking sites for ZAP-70, which is then activated and phosphorylates downstream molecules such as the linker for activation of T cells （LAT） and the SH2-domain-containing leukocyte protein of 76 kDa （SLP-76）. This cascade of events leads to T cell activation, proliferation, and differentiation. In addition, ZAP-70 expression in B cells is associated with chronic lymphocytic leukemia （CLL）, making it a potential biomarker and therapeutic target.