X-chromosome aneuploidies have long been associated with human cancers, but causality has not been established. In mammals, X-chromosome inactivation (XCI) is triggered by Xist RNA to equalize gene expression between the sexes. Here we delete Xist in the blood compartment of mice and demonstrate that mutant females develop a highly aggressive myeloproliferative neoplasm and myelodysplastic syndrome (mixed MPN/MDS) with 100% penetrance. Significant disease components include primary myelofibrosis, leukemia, histiocytic sarcoma, and vasculitis. Xist-deficient hematopoietic stem cells (HSC) show aberrant maturation and age-dependent loss. Reconstitution experiments indicate that MPN/MDS and myelofibrosis are of hematopoietic rather than stromal origin. We propose that Xist loss results in X-reactivation and consequent genome-wide changes that lead to cancer, thereby causally linking the X-chromosome to cancer in mice. Thus, Xist RNA is not only required to maintain XCI but also suppresses cancer in vivo. We carried out expression profiling in hematopoietic cells isolated from Xist-deficient female mice before disease (2 months old) and during myeloprolifeative neoplasm/myelodysplastic syndrome (MPN/MDS) and chronic myelomonocytic leukemia (CMML)-like disease (6 and 12 months old) and compared profiles of purified bone marow HSCs (KLS+CD34-), splenic B-cells (B220+), myeloid cells (CD11b+), and erythroid cells (Ter119+) against those of corresponding cell types from age-matched wild-type female mice. We conditionally targeted Xist locus in the blood compartment of mice by crossing Xist2lox/2lox mice (129Sv/Jae; Csankovszki et al., 1999) with Vav.Cre mice (B6.Cg-Tg (Vav1-cre)A2Kio/J; Jackson Laboratory). We used hematopoietic cells that were isolated from Xist-deficient and wild-type female progeny to analyze changes in gene expression due to loss of Xist.