Furthermore, functional studies have uncovered their roles in breast cancer as both tumour suppressor genes (eg miR-335) and oncogenes (eg miR-21). miRNAs deregulated in breast cancer influence the translational regulation of well-established regulatory molecules, such as oestrogen receptor-alpha, which is regulated by miR-206, and novel cancer-related molecules whose functions are not yet fully understood..
microRNA-21 was up-regulated in HER2 positive and Basal-like breast cancer types, while microRNA-206 was up-regulated in Luminal A and B types of breast cancer. microRNA-21 expression negatively correlated with the level of ER and PR but positively correlated with HER2 expression and tumor malignancy, while microRNA-206 showed the opposite trend.