Correlation of BIM expression levels with chemotherapy-induced apoptosis. (A) Seraphina cells were retrovirally transfected with p-MIRG-BIM and p-MIRG (empty vector). Single-cell clones were isolated from the BIM-expressing cell pools. Using quantitative RT-PCR and Western blot analyses, clones 1 and 2 showed intermediate and high BIM expression, respectively, with respect to the parental cell line. Seraphina clones 1 and 2 were incubated with individual chemotherapeutic agents, showing increased sensitivity to doxorubicin with respect to control cells, but not to other compounds tested (supplemental Figure 4). (B) Relative BIM expression of Seraphina clones 1 and 2 and other BL cell lines. (C) Chemotherapy treatment of clones 1 and 2 cells activated the intrinsic apoptotic pathway, through cleavage of caspase 9 and expression of PARP proteins, as determined by Western blot analysis. (D) Silencing of BIM by RNA interference in Ramos and KHM10B cell lines was correlated with an increase in the resistance profile to doxorubicin with respect to parental cell line and control (cells transfected with a scramble vector). (E) Kaplan-Meier survival curves of RAG2^−/−γc^−/− mice transplanted with wild-type and genetically manipulated Seraphina cells, which received chemotherapy or vehicle after tumor inoculation. Mice injected with Seraphina-transfected clones 1 and 2 cells but not Seraphina wild-type or cells transfected with the empty vector presented a statistically significant prolonged OS after treatment with doxorubicin compared with vehicle-treated recipients. (F) Follow-up studies of tumor metabolic activity by micro-PET performed 2 weeks after initiation of therapy revealed a better response after treatment in Seraphina clone 1 recipients than in clone 2 transplanted mice, whereas no response was observed in control mice carrying BL cells transfected with empty p-MIRG or parental wild-type cells.