Department of Haematology, Royal Bournemouth Hospital, UK.
ZAP-70 has emerged as a protein of potential prognostic importance in chronic lymphocytic leukemia (CLL) following gene expression profiling which compared the 2 well established prognostic sub-sets, those with unmutated and mutated IgVH genes. This protein tyrosine kinase (PTK), known to be of importance in T and NK cell signaling but absent in normal peripheral B cells, is expressed in the majority of the poorer prognosis unmutated CLL and absent in most cases with mutated IgVH genes. ZAP-70 has been shown to be functionally important in the CLL cases in which it is expressed; it is also important in B cell development in mice and there is preliminary evidence for its expression in human B cell progenitors and activated B cells. Whether its expression in a sub-set of CLL cases is a result of a more activated cell type or a reflection of the stage of maturation of the transforming event(s) in CLL is open to debate. ZAP-70 is expressed in a minority of other B cell tumors but correlation with IgVH gene mutational status is lacking. The problems with ZAP-70 measurement, which has yet to be standardized, are reviewed together with its current status as a prognostic marker in CLL.