IL21 and B-cell chronic lymphocytic leukemia: This is underscored by the observations that proliferating CLL cells in vivo are located close to activated helper T cells [9,10,11] and that in vitro CLL cells virtually never spontaneously divide [12,13] until they are provided with factors produced by helper T cells, such as CD40L, IL21, and IL4 [11,14,15].