Our results demonstrate that the combination of RA and Tz exerts a strong synergism in the inhibition of cell viability in human HER2-overexpressing breast cancer cells SKBR3 (ER+/HER2+/RARα+) and BT-474 (ER-/HER2+/RARα+), suggesting that this combination could be beneficial for both ER-positive and negative tumors. The gene discussed is ESR1; the disease is breast cancer.