Methyl-CpG-binding-protein 2 (MeCP2) is an abundantnuclear proteinexpressed in all cell types, especially neurons.19 Mutations in the X-chromosome-linked MECP2 gene cause Rett syndrome (RTT), a severe and incurable neurologicaldisorder that disproportionately affects young girls.20 Many potential RTT treatments are under development,21 but no disease modifying treatment yet exists.Two features of RTT etiology render therapeutic development especiallychallenging. The gene discussed is MECP2; the disease is atypical Rett syndrome.