Studies utilising CRISPR-based epigenome editing tools have focused almost exclusively on ex vivo cell culture systems to assess their epigenetic modalities. A major challenge in the field is the availability of clinically predictive model systems that can faithfully recapitulate relevant aspects of disease pathophysiology and allow for accurate assessment of novel therapies that can translate to successful human clinical trials. Utilising our privileged access to liver explants from two female paediatric patients with heterozygous X-linked OTC deficiency, we have had the opportunity to develop a humanised FRG mouse model from patient-derived hepatocytes with skewed X chromosome inactivation. Immunohistochemical and molecular analysis showed that X chromosome inactivation in the native liver of Patient OTC3 was skewed towards expression of the mutant OTC allele, while Patient OTC5 displayed a random X inactivation pattern. Dissociated patient hepatocytes were successfully engrafted in the FRG mouse to create chimeric human-mouse livers with clonal clusters of human hepatocytes that were either OTC positive or OTC negative. Quantitation by both immunohistochemical and FACS analyses confirmed that the skewing phenotype in the native patient liver was maintained following engraftment and expansion in the mouse liver. To provide a renewable source of skewed X inactivation model system in vivo, we successfully isolated the patient derived human hepatocytes from the primary FRG mouse recipient for repopulation into a second mouse recipient essentially providing an ongoing source to assess our therapeutic interventions. Our studies are currently in progress to assess CRISPR-dCas9-based epigenome targeting strategies with efficient rAAV gene delivery to reactivate the silenced functional OTC gene on the inactive X chromosome in female patient hepatocytes in vivo. The outcomes from this novel model system can be more broadly applied for a spectrum of X-linked disorders aiming at developing epigenetic therapies.