Furthermore, efficient in vitro transfection of miR-21i by the polyplexes induced apoptosis activation and strong anti-migratory effect in the treated cancer cells due to the downregulation of miR-21 expression and significant upregulation of miR-21 targets involved in the regulation of tumor proliferation, such as phosphatase and tensin homolog (PTEN), programmed cell death protein 4 (PDCD4), tumor protein p53 (p53), and caspase-3. This evidence concerns the gene PDCD4 and cancer.