Rett Syndrome (RTT) is a developmental disorder of the brain that occurs in females and is responsible for mental retardation and autistic behavior. Mutations in the MeCP2 protein situated on the X chromosome are the cause of this genetic disorder, and phenotypic variation ranging from mild to severe manifestations is observed in RTT. A major determinant of this clinical variability is the pattern of X-chromosome inactivation (XCI), a crucial epigenetic process that occurs early in development to balance the gene dose between XX females and XY males. Favorable XCI that silences the X chromosome with the mutant MeCP2 gene will lessen the severity of symptoms in girls with RTT. These researchers recently unraveled transcriptional circuitry linking XCI and mouse embryonic stem (ES) cell differentiation. They found that a pluripotent factor binds to the chromatin insulator Ctcf and is involved in X chromosome fate. The role of Ctcf and its protein partners will be investigated in the transcriptional regulation of MeCP2 in both normal and mutant MeCP2 female ES cells. X choice will be manipulated by altering the expression of the trans-factors crucial for this process. The long-term goal is to ask experimentally how one of the two female X chromosomes is chosen to be inactivated in normal development and to alter the fate of inactivation to the mutated X chromosome. Insight into how XCI is regulated will provide understanding in the pathogenesis of RTT that may offer hope in the amelioration of severe phenotypes.