Interleukin 21 (IL21) shows a pro-apoptotic effect in quiescent CLL cells [38] and B-lymphomas [65], but strongly potentiates the effects of CD40L/IL4 stimulation and induces CLL-cell division in most cases, independent of the clinico-biological characteristics of the patient cells [11,38,66,67]. This evidence concerns the gene IL4 and B-cell chronic lymphocytic leukemia.