PDCD4 and cancer: Through acting on a variety of tumor suppressors, such as PDCD4, PTEN, Maspin, BMPRII, and Sprouty2, the dysregulated miR-21 participates in the proliferation, invasion, metastasis, antiapoptosis, and other biological behaviors of malignant tumor cells; miR-21 plays a role similar to oncogenes in the occurrence and growth of malignant tumors, and can be developed as a potential target for the treatment of malignant tumors.