In breast cancer, the anti-proliferative activity of ATRA is observed predominantly in tumors characterized by a luminal and ER+ (Estrogen Receptor positive) phenotype, while the basal and HER2+ (Human Epidermal Growth Factor Receptor 2 positive) subtypes of this neoplastic disease are generally resistant to the retinoid [22, 31, 32]. Here, ESR1 is linked to breast cancer.