PPARD and cancer: Our results suggest PPARδ allows breast cancer cells to ‘endure' harsh metabolic conditions (Figures 2,3 and 4), analogous to its ability to promote endurance in muscle cells and prevent exhaustion of stem cells.4, 5, 6 Taken together, the observations suggest that PPARδ drives aggressive clinical behavior because it allows cancer cells to grow in metabolically stressful conditions, which would include the presence of chemotherapies that cause ER stress (Figure 3d).1, 7, 8