GSTP1 and head and neck squamous cell carcinoma: Aberrant DNA methylation was assessed for four genes—p16 (CDKN2A), O6-methylgua-nine-DNA-methyltransferase (MGMT), glutathione S-transferase P1 (GSTP1), and death-associated protein kinase (DAP-kinase)—in both primary tumor and serum of 50 HNSCC patients.