XIST and breast cancer: As summarized in Figure 1A, in our previous study [16] the analysis of XCI and XIST RNA in human mammary epithelial cells (HMEC) and in human breast cancer cell lines revealed three distinct patterns: presence of both Xa and Xi and XIST expression (normal, type 0); Xa replication, Xi loss and no XIST expression (type 1); Xa replication, Xi loss and XIST expression (type 2).