CDKN2A and esophageal cancer: CDKN2A, cyclin‐dependent kinase inhibitor 2A; EC, esophageal cancer; EGFR, endothelial growth factor receptor; HER2, human epidermal growth factor receptor 2; mTOR, mammalian target of rapamycin; PI3K, phosphatidylinositol 3‐kinase; TP53, tumor protein 53.