Interestingly, basal ROS levels in butein-resistant and luminal HER2+ HCC-1419, HCC-2218 and SKBR-3 breast cancer cells were lower than those in butein-sensitive breast cancer cells including HER2+ BT-474 (luminal, ER+/PR+/HER2+) and HCC-1569 (basal A, HER2+) cells (Figure 3A). Here, ESR1 is linked to breast cancer.