CST6 and breast cancer: Analysis of the CST6 gene shows a single CpG island with many potential methylation sites in the promoter and the exon 1 of the gene (~64 CpGs in a 507 bp segment) [23] and it was recently shown that this region is a target for DNA methylation, which results in loss of cystatin M expression in breast cancer lines and breast carcinomas [23-25].