ESR1 and cancer: In addition to topoisomerase I inhibitors, both T47D/SN120 and T47D/SN150 sublines were cross-resistant to various anti-cancer drugs that are used in breast cancer treatment, including microtubule inhibitors (paclitaxel and vinblastine), anti-metabolites (5-fluorouracil), topoisomerase II inhibitors (doxorubicin and mitoxantrone), estrogen receptor blockers (endoxifen), and a tyrosine kinase inhibitor (gefitinib) (Table 2).