ESR1 and breast cancer: In addition, although both receptors facilitate chemoresistance to the selective estrogen receptor modulator (SERM) tamoxifen in breast cancer, resistance to tamoxifen is characterized by decreased expression of IGF-1R and increased EGFR expression, and more surprisingly, no detectable difference was found in EGFR expression between breast cancer cells resistant and sensitive to tamoxifen when measured with cytometry (79).