As examples, methylation of the CDKN2A promoter has been detected in sputum of smokers up to 3 years before they were diagnosed with cancer [19, 20], GSTP1 has been used in prostate cancer detection [21], and RASSF1A can be detected in serum from gastric colorectal adenocarcinoma patients [22]. Here, GSTP1 is linked to cancer.