ESR1 and breast cancer: Several studies found a correlation between the hypermethylation of cytosine guanine phosphodiesters (CpG dinucleotides) of the ESR1 promoter and decreased ER expression in breast cancer tissue [11,12,13,14,15,16], and few studies also found a correlation of ESR1 promoter methylation with outcomes of endocrine treatment [11,17].