To explore the molecular mechanisms underlying ERRα-mediated breast cancer aggressiveness, we initially examined the expression of ERRα and its coactivator PGC-1α at the protein level in two human breast cancer cell lines with distinct ER statuses, MCF-7 (ER-positive) and SK-BR-3 (ER-negative), under different oxygenation conditions. This evidence concerns the gene ESR1 and breast carcinoma.