Zeta-chain associated protein kinase, ZAP70, is a 70 kDa member of the Syk family kinase predominantly involved in T cell receptor (TCR) signaling. It is structurally homologous to Syk, a PTK that is involved in proximal BCR signaling. ZAP-70 is a key signaling molecule in T cell activation and also plays a role in apoptosis and cell migration. SYK family tyrosine kinases contain a C-terminal kinase domain and tandem N-terminal SH2 domains that bind phosphorylated ITAMs (immunoreceptor tyrosine-based activation motif). Linker region that contains multiple tyrosines separates the SH2 domains from the kinase domain. Phosphorylated tyrosines act as docking sites for phospholipase Cγ1 (PLCγ1). ZAP-70 and Syk are functionally homologous in antigen receptor signaling. Expression of ZAP-70 in Syk− B cells reconstitutes SCR function. Reconstitution requires the presence of functional Src homology 2 (SH2) and catalytic domains of ZAP-70. Expression of ZAP-70 is an important negative prognostic factor in chronic lymphocytic leukemia (CLL) with more rapid disease progression and shorter survival.