In fact, by deleting Xist in vivo in murine hematopoietic stem cells after the occurrence of X inactivation, Yildrim et al. were able to induce highly aggressive lethal myeloproliferative neoplasm and myelodysplastic syndrome (MPN/MDS), with 100% penetrance, in both the female homozygous and heterozygous conditions, whereas male mutants remained healthy and viable throughout the 2-year observation period [32]. The gene discussed is XIST; the disease is myeloproliferative neoplasm.