ZAP70 and B-cell chronic lymphocytic leukemia: In particular, gefitinib may have an effect in the lymph node microenvironments where BCR signaling occurs27 and ZAP-70 expression is upregulated.28 It is important to note that the complexity of feedback loops and interactions of ZAP-70 in CLL cells are not clearly understood, making it difficult to definitively determine the precise action of gefitinib.