AAV vectors expressing Cas9 and sgRNA targeting a single gene (Mecp2, which causes the neurodevelopmental disorder Rett syndrome), as well as multiple genes (Dnmt1, Dnmt3a and Dnmt3b from the DNA methyltransferase family), in adult mouse brains have been stereotactically injected into the dentate gyrus, enabling reverse genetic studies of gene function in the brain [34]. The gene discussed is MECP2; the disease is atypical Rett syndrome.