PIWIL1 and lung adenocarcinoma: Since the activation of CT genes in tumors is mediated largely by alterations in DNA hypomethylation 23, we first analyzed the relationship between DNA methylation levels of the 2 kb upstream of PIWIL1 and its expression levels in 426 lung adenocarcinoma patients based on TCGA database, and found a significantly negative correlation (ρ = −0.286, P = 1.883 × 10−9, Fig. 3B).