Alterations in DNA methylation may cause disturbances in regulation of gene expression, including drug metabolism and distribution. Moreover, many cancers, including breast cancer, are characterized by DNA hypomethylation and a decreased 5-hydroxymethylcytosine level. The abnormal cell growth found in breast carcinoma might be the result of impaired up-regulation of breast cancer receptors. Receptors’ expression in breast cancer determines clinical outcome, and it is possible that they lead to different DNA methylation patterns. Excessive steroid exposure can affect DNA methylation by promoting demethylation of CpG islands in promoter regions of genes, and hence may have an impact on promotion and progression of breast cancer cells. Tamoxifen, as a leading drug in breast cancer hormone therapy, has an ability to act like estrogen or antiestrogen depending on the type and localization of the breast cancer receptor. Further studies are needed to determine whether tamoxifen, similarly to steroids, may evoke changes in methylation pattern.