CST6 and breast cancer: The loss of cystatin M/E expression has been strongly correlated with CST6 promoter hypermethylation in a panel of breast cancer cell lines (MDA-MB-231, ZR75-1, MCF-7, T47D, BT-549, Hs578T, MDA-MB-436 and MDA-MB-453) and in primary breast carcinomas, indicating that this aberrant characteristic is common in breast malignancies [77,78,79].