THBS1 and gastric cancer: Five different classes of methylation behaviours were observed: i) genes methylated in gastric cancer only (GSTP1 and RASSF1A), ii) genes showing a significantly higher methylation frequency in gastric cancer than in other lesions (COX-2, hMLH1 and p16), iii) genes with a high and similar methylation frequency in all four lesions (APC and E-cadherin), iv) a gene with a low and similar methylation frequency in four-step lesions (MGMT), and v) genes showing an increasing methylation frequency during the progression of the disease (DAPK, p14, THBS1 and TIMP-3).