For instance, the methylation of PCDH19 predicted a poor prognosis of hepatocellular carcinoma [31]; the methylation of DFNA5 showed strong potential as a prognostic biomarker for breast cancer; and the signature of CXCL11 combined with HMGA2 could precisely predict the OS of patients with high-grade serous ovarian cancer [30]. This evidence concerns the gene PCDH19 and breast cancer.