Nurse-like cells [11], bone marrow stromal cells [12], [13], T cells [14], [15], dendritic cells [16], accessory leukocytes [17], the leukemic cells themselves [18] and the cytokines derived from them: BAFF and APRIL [19], IL-6 [20], IL-10 [21], IL-2 [22], IL-4 [23], IL-21 [24], IL-12 [25] and IL-15 [26], [27], have all been suggested to act as pro-survival factors or growth factors in CLL. This evidence concerns the gene IL21 and B-cell chronic lymphocytic leukemia.