Addiction to IRF4 in MCL cells. (A) The MCL (JEKO and REC1), MM (H929), and GCB-DLBCL (OCI-LY19) cell lines were retrovirally transfected with a doxycycline-inducible, IRF4-targeting shRNA plasmid containing GFP. shRNA expression was induced by the addition of doxycycline (50 ng/mL) starting on day 0. The percentage of GFP⁺ cells was monitored by flow cytometry as a measure of live cells expressing shIRF4, and normalized to baseline (day 0). Four days after the addition of doxycycline, IRF4 knockdown was assessed by Western blot, normalized to TBP, and expressed as the ratio between cells transduced with shcontrol and shIRF4. (B) Expression of IRF4 coding region (open symbols) or the corresponding empty vector (black symbols) in REC1 (squares) and H929 (triangles) cells also transfected with shIRF4. Induction of shRNA expression and monitoring of transfected cells was as in (A). (C) REC1 cells expressing shcontrol or shIRF4 were selected with puromycin and induced for 4 days with doxycycline. After 24 hours with increasing bortezomib (Bzm) doses, cytotoxicity was analyzed by MTT.