Epigenetic control of gene expression is a fundamental process important in controlling biological processes in multicellular organisms; epigenetic marks are laid down throughout development and are needed for cellular identity and maintenance, the failure of which can result in various diseases, including cancer. In females X chromosome inactivation (XCI) is a prominent epigenetic mechanism that inactivates majority of genes on one of the two X chromosomes, as a form of dosage compensation. We use XCI as a model system to identify novel epigenetic modifiers in vitro. During XCI gene silencing happens in three main stages: initiation, establishment and maintenance. We have previously identified epigenetic modifiers involved in the initiation and establishment phase of XCI using mouse embryonic stem cells (mESCs). Here we use female mEpiSCs (mouse epiblast stem cells) isolated from E6.5 embryos to perform short hairpin RNA (shRNA) screens to identify novel epigenetic modifiers in the maintenance of XCI. Female mEpiSCs are primed cells that have undergone the initiation and establishment phases of XCI and appear to stably maintain XCI, however not all events associated with the maintenance phase of XCI are complete, such as methylation of CpG islands. This means that the inactive X (Xi) is likely more amenable to reactivation, and therefore mEpiSCs are an ideal cellular system for screening for factors other than DNA methyltransferase 1 that are involved in maintenance. One other factor important in the maintenance of XCI is Smchd1 (Structural maintenance of chromosomes flexible hinge domain containing 1). Concurrent with our screens in mEpiSCs, we are using Fluorescence Recovery After Photobleaching (FRAP) to analyse the stability of Smchd1 binding to the inactive X chromosome in cells in the maintenance phase of silencing. We produced female mouse embryonic fibroblasts (MEFs) carrying a Smchd1-GFP knock-in allele, where we observe Smchd1 in 3 main regions: the Xi, autosomal loci, and the nucleoplasm. Our preliminary data suggests that Smchd1-GFP is less dynamic on the Xi compared with other nuclear regions, which may relate to the overall heterochromatin compaction of the Xi.