PDCD4 and esophageal squamous cell carcinoma: MiR-21-5p is known to be overexpressed in ESCC tissues [22,23] and can promote the growth, proliferation, invasion, and metastasis of ESCC cells by targeting multiple tumor suppressor genes, including programmed cell death protein 4 (PDCD4), PTEN, and Fas ligand (FasL) [24,25,26].