BTK and B-cell chronic lymphocytic leukemia: Moreover, IL-9 expression was reduced in 4 CLL patients showing a significant response to second line Ibrutinib treatment (range follow-up 26.2 ± 7.3 months) (Fig. 7F, Supplementary Table 4), suggesting that the response of CLL patients to BTK inhibitors results, at least in part, from the normalization of IL-9 release by leukemic cells.