PDCD4 and digestive system neoplasm: miR-21-5p promotes peritoneal metastasis through EMT in gastric cancer, increases the proliferation, migration, and invasion of colon cancer by downregulating Tiam1, promotes cell migration and invasion in esophageal cancer by targeting PDCD4, induces metastasis of human cervical carcinoma cells, and can be used as a biomarker to predict the recurrence of digestive system tumors [18, 19].