TP53 and cancer: Aberrant DNA promoter methylation of tumor suppresser genes, including hypermethylation of p16 [27,28], APC [29] and RASSF1A [28,30], and hypomethylation of p53 [31], has been observed in blood leukocyte DNA from lung cancer patients, suggesting that PBL DNA methylation may serve as a cancer-related biomarker.