miR-27a is one of the earliest examples of drug resistance, since its downregulation enhances the sensitivity of ESCC cell lines to both P-gp and non–P-gp substrate drugs, potentiates ADR-induced apoptosis, and reduces P-gp, BCL2, and MDR1 levels while increasing BAX, shifting the BCL2/BAX ratio toward apoptosis [160]. Here, PGP is linked to esophageal squamous cell carcinoma.