CHD5 and breast cancer: Strikingly, the four cell lines with the most extensive methylation (BT-20, MDA-MB-231, ZR-75-30 and MDA-MB-175) had barely detectable CHD5 mRNA expression, and only one of the four cell lines, ZR-75-30, had a copy number loss at CHD5 (Additional file 1 Table S4), suggesting that promoter methylation is more often responsible for the down-regulation of CHD5 in some breast cancers.