XIST and fibrosarcoma: The XIST lncRNA is unique in its ability to initiate the establishment of an inactive chromosome, and remarkably induction of an XIST cDNA in human somatic cells is able to recapitulate silencing and establish many of the heterochromatic features of the inactive X. While mouse embryonic stem cells are reported to have a limited developmental window during which Xist induces inactivation (reviewed in 3), we have previously demonstrated that both these HT1080-fibrosarcoma derived cells as well as the near triploid 293 cells were able to induce silencing and many features of an inactive X [17].