ABCB1 and breast cancer: In order to get a broader picture of the prevalence of pre-neoplastic DNA methylation changes in tumor-surrounding tissues in breast cancer patients, we included methylation levels that we had obtained for the same set of breast tissue samples in previous studies (CCND2, DAPK1, GSTP1, HIN-1, MGMT and RASSF1A: [24]; ABCB1, ABCC1 and ABCG2: [31]).