Mutation in the transmembrane domain of BAX abolished cell death induced by ABT-199. (A) Scheme showing the generation of HBL2R cells. A detailed description is provided in “Methods.” (B) Both HBL2 and HBL2R cells cultured without ABT-199 for 4 weeks (labeled as HBL2SA) were resistant to the BH3 mimetics. (C) HBL2R cells did not show activation of the mitochondrial apoptotic pathway induced by the BH3 mimetics, as shown by the annexin V-FITC FACS and the lack of caspases 3 and 9 cleavage. Experiments were performed in duplicate. (D) Kaplan-Meier overall survival curves for Rag2^−/−IL2γc^−/− mice transplanted with HBL2 or HBL2R cells. (E) A representative image showing differences in the tumor volume in mice inoculated with HBL2 and HBL2R cells and treated with ABT-199 (marked with arrows). (F-G) Expression profiling of BCL2 family proteins did not show changes between HBL2 and HBL2SA cells. Shown are representative data from 1 of 3 independent experiments. (H) Cells with acquired resistance to ABT-199 showed a missense mutation in BAX transmembrane domain (G179E). Arrows indicate the base change. cHBL2R, cDNA; gHBL2R, genomic DNA.