ZAP70 and B-cell chronic lymphocytic leukemia: Notably, the expression of ZAP-70 in CLL cells frequently varies across the entire clone and a somewhat arbitrary threshold of >20% is required to classify a patient by flow-cytometry as “ZAP-70-positive.” Importantly, the expression levels of ZAP-70 in CLL cells are relatively stable over time (6, 10, 12).