Gastric carcinoma and chronic gastritis: Epigenetic regulation of CDH1 (E-cadherin), CDKN2A (p16 <sup>INK4A</sup>), PTGS2 (COX-2) and EGFR genes through methylation,Carcinoma gástrico e gastrite crónica: Regulação epigenética por metilação dos genes CDH1 (caderina-E), CDKN2A (p16 <sup>INK4A</sup>), PTGS2 (COX2) e EGFR