The present invention provides a method for diagnosing and determining prognosis of certain cancers (e.g., esophageal squamous cell carcinoma or ESCC) in a subject by detecting suppressed expression of the DLEC1 gene, which in some cases is due to elevated methylation level in the genomic sequence of this gene. A kit and device useful for such a method are also provided. In addition, the present invention provides a method for treating cancer by increasing DLEC1 gene expression or activity.