ESR1 and breast cancer: However, ER+ breast cancer is exposed toa complex milieu of hormones, growth factors and other stimuli, and the study of asingle-nuclear receptor, such as ER, in the absence of all hormones other than itscognate ligand (oestrogen) does not accurately reflect the physiological situations.Recent findings have shown a substantial degree of nuclear receptor crosstalk inER+ breast cancer, with both PR and AR converging on the ER pathway(9, 67, 68, 69, 70).