In vitro studies indicated that miR-21-5p/PDCD4 signaling promotes cell proliferation in MCF-7 luminal-like breast cancer cells and induces the invasion and metastasis of MDA-MB-231 triple-negative breast cancer cells (Frankel et al., 2008; Zhu et al., 2008). The gene discussed is PDCD4; the disease is breast carcinoma.