NNMT and esophageal squamous cell carcinoma: Integrating analyses of scRNA-seq and metabolomic data showed that the highly expressed nicotinamide N-methyltransferase (NNMT) is the key enzyme contributing to the metabolic changes, which decreases H3K4me3 and m6A modification of E-cadherin, thereby promoting epithelial–mesenchymal transition (EMT) and metastasis ability of ESCC cells.