Cystatin M, originally described as a putative tumor suppressor, whose expression is often diminished or completely lost in metastatic breast cancers [15] has been clearly shown to be epigenetically regulated by strong hypermethylation of the CST6 gene promoter in breast cancer cell lines [23], in breast cancer and metastatic lesions in the lymph nodes [34], in malignant gliomas [36], in cervical [37] and prostate cancer [38]. The gene discussed is CST6; the disease is breast carcinoma.