The impact of IL-4 and PF-956980 on the IC₅₀ values of cytotoxic agents toward CLL cells. IC₅₀ values for fludarabine, chlorambucil, and nutlin-3 were determined (“Methods”) in the presence of no further additions, 10 ng/mL IL-4, 1μM PF-956980, or IL-4 plus PF-956980, as indicated. Data for each treatment group are presented as mean values + standard errors. Statistical significance of the differences between means was assessed by computing P values, as indicated (2-tailed Student t test for paired samples). Isolates from patients 1–16 were used in panel B, with the omission of patient 8 in panel A and patients 4, 8, and 14 in panel C.