Flow cytometry for ZAP-70: New colors for chronic lymphocytic leukemia.

ZAP-70 has become one of the most studied prognostic markers in Chronic Lymphocytic Leukemia (CLL). ZAP-70 is remarkable in many ways: ZAP-70 has been identified as the best discriminating gene between prognostically distinct CLL subtypes using large scale gene expression profiling; ZAP-70 has been shown to enhance signal transduction in CLL B-cells and therefore could contribute to disease progression; and ZAP-70 is one of the rare examples of an intracellular target considered for clinical flow cytometry. This issue attests to the enormous effort and the steady progress made in overcoming technical challenges of testing for ZAP-70 expression and sets the foundation for a successful translation of this important marker into clinical practice. Despite the best effort, one will likely have to accept that not all cases can be clearly assigned to one or the other group, given that ZAP-70 expression between CLL patients falls along a continuum from absent to high. Nevertheless, ZAP-70 expression could become a key parameter to guide patients towards risk adapted treatment strategies in prospective clinical trials.