XIST and breast cancer: The mechanism by which XIST functions in breast cancer is controversial, with some studies showing interaction of the tumor suppressor gene BRCA1 with XIST, and that loss of BRCA1 is associated with loss of XIST. Upon reactivation of BRCA1, XIST staining by in situ hybridization was restored (Ganesan et al., 2002, 2004).