The detection of the ZAP-70 gene product by flow cytometry, however, is not easy to be performed, since it requires cell membrane permeabilization and the simultaneous use of T cell markers to discriminate the expression of ZAP-70 protein between malignant B-CLL cells and residual T lymphocytes [14]. The gene discussed is ZAP70; the disease is B-cell chronic lymphocytic leukemia.