Mammalian X-linked gene expression is highly regulated as female cells contain two and male one X chromosome (X). To adjust the X gene dosage between genders, female mouse preimplantation embryos undergo an imprinted form of X chromosome inactivation (iXCI) that requires both (also known as Rnf12) and the long non-coding RNA . Moreover, it is thought that gene expression from the single active X is upregulated to correct for bi-allelic autosomal (A) gene expression. We have combined mouse genetics with RNA-seq on single mouse embryos to investigate functions of on the temporal regulation of iXCI and . Our results reveal crucial roles of for the maintenance of high RNA levels, clouds and X-silencing in female embryos at blastocyst stages, while initial expression appears -independent. We find further that X/A upregulation is initiated in early male and female preimplantation embryos.