ESR1 and breast cancer: We have previously published that a. ERRγ is upregulated during the acquisition of Tamoxifen (TAM) resistance by ER+ breast cancer cells and b. overexpression of ERRγ is sufficient to induce TAM resistance [19], and c. ERRγ’s transcriptional activity and ability to induce TAM resistance is enhanced by activation of the ERK/MAPK pathway [20].