ESR1 and breast cancer: We discovered that AI-resistant MCF-7:5C breast cancer cells constitutively express enhanced ERα (total and phosphorylated) expression as well as ER-regulated genes compared to AI-sensitive MCF-7 and T47D cells and that blockade of IFNα signaling or knockdown of STAT1/2 markedly reduces ERα expression and ER-regulated genes in these cells.