ESR1 and breast cancer: We focused our analysis on DNA methylation of estrogen receptor genes (ESR1, ESR2) and the tumor suppressor genes TP53, CDKN2A, BRCA1, BRCA2 which play a relevant role in key cellular processes such as cell growth, apoptosis, and DNA repair and have been associated (with different mechanisms) with increased breast cancer risk [21,22,23,24]; methylation of repetitive elements (LINE-1, Alu); and telomere length.