PDCD4 and esophageal squamous cell carcinoma: Aberrantly expressed miRNAs promote ESCC development through regulation of the expression of their target genes, such as PTEN (phosphatase and tensin homolog) and PDCD4 (programmed cell death 4), NF-κB, FBXW7 (F-box and WD repeat domain containing 7), E-cadherin and SOX6.