GSTP1 and head and neck squamous cell carcinoma: Other findings on methylation patterns in HNSCC previously described in the literature include associations between levels of primary tumor and ctDNA gene methylation levels, hypomethylation of Alu elements in ctDNA samples of HNSCC patients, and hypermethylation of gene promoter sequences in ctDNA of HNSCC patients, including the promoter sequences of CDKN2A, CDKN2B, DAPK1, MGMT, GSTP1, PRDM2, RASSF1, DLEC1, UCHL1, RARβ2, WIF1, DCC, MLH1, and CDH1 [28].