IL21 and B-cell chronic lymphocytic leukemia: Consistently, treatment with the BH3 mimetic A‐1331852 specifically targeting Bcl‐XL, normalized the differences in venetoclax sensitivity between CD40‐activated CLL cells stimulated with or without cytokines, confirming by pharmacological means that the opposing effects of IL‐21 and IL‐4 are mediated via Bcl‐XL (Fig. 1C,D).