CDKN2A and esophageal squamous cell carcinoma: Unlike highly penetrant germline mutations causing ‘hereditary’ cancers, such as CDKN2A mutations in melanoma and BRCA1 mutations in breast cancer, lung cancer and ESCC belong to ‘sporadic’ (‘non-hereditary’) malignant phenotype but a number of recent studies of heritability assessment suggests that these phenotypes (include twelve cancer types) of sporadic tumors share common genetic variants [24].