ESR1 and breast cancer: For instance, the triple-negative breast cancer (TNBC) cells—which are malignant and defined as a subtype of breast cancer cells that are the absence of estrogen receptors (ER), progesterone receptors (PR), and human epidermal growth factor receptor−2 (HER2)—exhibited a highly glycolytic phenotype with high glucose uptake, increased lactate production, and low mitochondrial respiration compared with ER-positive cells [51].