Resistance to apoptosis induced by fludarabine has previously been described in CLL cells cultured with CD40L-expressing cells in the presence of IL-4.[35] The mechanism(s) for this have not been fully elucidated but the upregulation of Bcl-2 may well be a contributory factor.[33] In contrast, NVP-AUY922-AG was equipotent in IL-4 supplemented cultures suggesting that its mechanism of killing was distinct from fludarabine. This evidence concerns the gene IL4 and B-cell chronic lymphocytic leukemia.