BRCA1 and breast cancer: The same discrepancy of XIST and BRCA1 is observed in human breast tumors, with some studies showing concurrent loss of both BRCA1 and XIST in highly aggressive breast cancers (Richardson et al., 2006) while other studies of breast cancer tissue from patients with germline BRCA1 mutations show expression of XIST in a significant number of samples (Vincent-Salomon et al., 2007).