Supplementary Figure 1: Generation and characterization of resistant cell pools. Supplementary Figure 2: Copy number assessment of receptor tyrosine kinases in resistant cell pools. Supplementary Figure 3: Trastuzumab resistance is reversible. Supplementary Figure 4: Drug resistant cells are more sensitive to OXPHOS inhibitors than parental cells. Supplementary Figure 5: Drug resistant cells have similar mitochondrial DNA copy numbers and ATP levels as the parental cells. Supplementary Figure 6: Glycolysis in parental BT474 versus trastuzumab resistant cells. Supplementary Figure 7: ATP synthase gene expression in BT474, SKBR3 cells and their derived trastuzumab-resistant cells. Supplementary Figure 8: Cumulative survival of breast cancer (all, basal, luminal) patients with patients divided in half by ATP5B (left) or ATP5A1 (right) mRNA level. Supplementary Figure 9: ATP5B protein expression correlation with survival in HER2+, trastuzumab treated patients from HeCOG 10/05. Supplementary Figure 10: ATP5B is required for the maintenance of resistance. Supplementary Figure 11: BT-TR2 tumor morphology and proliferation. Supplementary Table 1: Sequences of primers and plasmid inserts. Supplementary Methods Supplementary References