sentence1: Consequently miRNAs have been demonstrated to act either as oncogenes (e.g., miR-155, miR-17−5p and miR-21)  or tumor suppressors (e.g., miR-34, miR-15a, miR-16−1 and let-7)
sentence2: The extent to which miRNAs specifically affect metastasis remains unclear, as all the miRNAs reported to affect metastasis also exert potentially confounding influences on primary tumor development, apoptosis, and/or cell proliferation