In line with these observations, Patten et al. have evidenced a phenotypic heterogeneity in luminal breast cancer patients, which were related to the epigenetically-regulated YY1 transcription factor that was shown to be a critical determinant of ERα transcriptional activity promoting tumor growth in most luminal patients and also contributing to the expression of genes mediating resistance to endocrine treatment [267]. This evidence concerns the gene ESR1 and breast cancer.