PDCD4 and breast cancer: Since PDCD4 downregulation is a major marker of poor prognosis and endocrine therapy resistance in ER+ breast cancer patients [49], the pharmacological, as well as non-pharmacological, suppression of miR-21 represents a promising treatment strategy for ER+ breast cancer patients; however, questions remain to be addressed regarding the regulatory mechanisms that control miR-21 expression during breast carcinogenesis, as well as the mechanism by which exercise training alters miR-21 expression.