ZAP-70, a Syk-family protein tyrosine kinase, plays a critical role in mediating T-cell signal transduction in response to T-cell antigen receptor (TCR) activation. It is primarily expressed in T cells and natural killer (NK) cells. It also labels mast cells, basophils and pro/pre B cells, but not mature B cells. ZAP-70 antibody is useful in identification of the subtype of chronic lymphocytic leukemia (CLL). ZAP-70 is positive in CLL with mutation of the immunoglobulin heavy-chain variable region (IgVH) genes, but negative in CLL without IgVH mutation. ZAP-70 expression is associated with disease progression in CLL.