XIST and breast cancer: To verify the hypothesis that the lack of functional BRCA1 is involved in the inappropriate expression of XIST from Xa, we performed BRCA1 silencing in HMEC (XCI-type 0) and in the following BRCA1wt breast cancer cell lines: MDA MB 231(XCI-type 1), T47D (XCI-type 1) and MCF7 (XCI-type 2).