ESR1 and breast cancer: Overall, the data reported here indicate that OU activates the 26S proteasome, prevents cell proliferation, and induces ERα degradation in cells modeling the LumB BC phenotype, thus demonstrating that OU works in LumB ERα-positive BC cell lines as we already reported in two LumA BC cell lines (i.e., MCF-7 and ZR-75-1 cells) [4] and further suggest that the increased anti-proliferative activity of this CG in ERα-positive cell lines with respect to ERα-negative cell lines could be due to OU-induced receptor degradation and 26S proteasome activation.