EGR1 mediates ibrutinib resistance. (A) Retroviral EGR1 expression after induction with 20 ng/mL doxycycline (Dox; left). Trypan blue cell viability assay after 3-day ibrutinib treatment with or without induction of EGR1 expression by 20 ng/mL Dox (right). Error bars represent mean ± SD of triplicates (∗P < .05; ∗∗P < .01; ∗∗∗P < .001; ∗∗∗∗P < .0001). (B) Flow cytometric analysis of cell viability of GFP-positive shRNA-expressing cells for up to 9 days of EGR1 shRNA or control shRNA expression in the presence of ibrutinib or DMSO control. Error bars represent mean ± SD of triplicates (∗∗∗P < .001; ∗∗∗∗P < .0001). (C) Ibrutinib-resistant ABC DLBCL xenografts. OCI-Ly10 ibrutinib-resistant cells were established as a subcutaneous tumor (average, 150 mm³) in NSG mice, and then treated with 3 mg/kg ibrutinib (intraperitoneally.) daily for 4 weeks or PBS vehicle. EGR1 shRNA or control shRNA expression was induced with 2 mg/mL Dox in drinking water. Tumor volume and the survival of recipient mice in each group are shown. Error bars represent mean ± standard error of the mean (SEM); two-way analysis of variance [ANOVA], ∗P < .05; ∗∗P < .01; ∗∗∗P < .001).