MGMT and esophageal squamous cell carcinoma: The diagnostic performance of 13 potential methylation markers, CDKN2A, CDKN2B, TFF1, MGMT, MLH1, DAPK1, SCGB3A1, TFPI2, DACH1, SOX17, CHFR, CDH1, APC that have been reported in ESCC were also evaluated using the same approach in TCGA ESCC dataset (Additional file 1: Fig. S5).