[6,13] Wei et al found that by treating the tamoxifen-sensitive breast cancer cell line MCF-7 with the exosome secreted by chemotherapy-resistant cell line MCF-7TamR, it can acquire drug resistance, because the miR-221/222 in the exosome secreted by the drug-resistant cell line inhibited the expression of the estrogen receptor α target gene (ERα). This evidence concerns the gene ESR1 and breast cancer.