Quantification of 5mC, 5hmC and 5fC levels in GCC cell lines as well as qRT-PCR/cDNA microarray analysis of GCC cell lines and tissues suggest that GCCs preferentially utilize the oxidation pathway for ADD, involving TET1. Furthermore, we could find no mutation of the IDH1 (IDH1R132) and IDH2 (IDH2R172) genes in GCC cell lines, which are associated with aberrant production of the TET inhibiting oncometabolite 2-hydroxyglutarate. This evidence concerns the gene IDH1 and goblet cell carcinoma.