Differences in the quantity of DNA found in the urine and saliva of smokers versus nonsmokers: implications for the timing of epigenetic events.

TP53 is a tumor-suppressor gene coding for p53, a protein responsible for cell-cycle arrest and DNA repair. Smoking has been demonstrated to lead to the methylation of tumor-suppressor genes in noncancerous lung biopsy tissues of smokers, and in bodily fluids, promoter hypermethylation occurs very early in the progression of cancer. Thus, DNA methylation changes may be initiated long before cells become cancerous.