CDKN2A and esophageal squamous cell carcinoma: Previous studies suggested that some hypermethylated genes occurred in early-stage ESCC, including the well-known genes MGMT [19], CDKN2A [20], MLH1 [21], and CDH1 [22], while some are hypermethylated in late-stage such as CHFR [23], and others, such as hypermethylated APC are not associated with ESCC stage [24].