Although both luminal estrogen receptor (ER)α-positive (MCF-7 and T47D cells) and claudin-low triple negative breast cancer (TNBC) (MDA-MB-231 and BT549 cells) cell lines express PPARγ, higher protein levels of this receptor were found in the TNBC cell lines than in the luminal subtype lines [10], suggesting that PPARγ may represent a molecular target for treatment of the more aggressive breast cancer phenotype. This evidence concerns the gene ESR1 and breast cancer.