ESR1 and breast cancer: For ER-/HER2- breast cancer, Lee et al. postulated the activities of microRNA let-7d and miR-18a as possible prognostic factors and confirmed this assumption in two independent ER-/HER2- breast cancer gene expression databases: the overactive let-7d was associated with a better overall survival rate, whereas in one dataset, the activities of let-7d were related to the metastatic process [34].