The expression of ZAP-70 in B cell malignancies was first detected in CLL with 20–80% of leukemic B cells having ZAP-70 expression levels equivalent to autologous CD3+ T cells in patients, correlating with unmutated IGHV gene and poor clinical outcomes (5, 6, 10, 11). Here, ZAP70 is linked to B-cell chronic lymphocytic leukemia.