In patients’ samples of the less aggressive BC subtypes (luminal A, B HER2-), a significant increase in the H3K27me3 mark was detected at promoters of genes encoding hormone receptors ERS1, −2 (estrogen receptor 1, 2) and PGR (progesterone receptor) compared with luminal B HER2+, HER2+, and basal-like subtypes, in which a decreased H3K27me3 (and an increased level of acetylated marks) was found at the promoters of the genes encoding histones’ modifying enzymes EZH2, P300 (histone acetyltransferase) and SRC3 (steroid receptor coactivator 3), and tumour suppressor BRCA1 [46]. This evidence concerns the gene ESR1 and breast cancer.