In 1999, mutations in the epigenetic factor methyl-CpG binding protein 2 (MECP2) were identified as an underlying cause of Rett syndrome (RTT) in girls (Amir et al., 1999). Since that time, studies from many different investigators have been directed toward understanding the in vitro and in vivo mechanisms of MeCP2 action and the consequences of MeCP2 loss in different mouse models (Guy et al., 2011). Experiments provide support for MeCP2's role as transcriptional repressor, activator, and RNA-binding protein, among other functions, but, to date, no unifying principle to explain MeCP2 function has emerged. Further, RNA profiling of MeCP2-deficient mouse and human brains shows hundreds of changed transcripts (Colantuoni et al., 2001; Tudor et al., 2002; Chahrour et al., 2008). Possibly for these reasons, no single therapeutic has been shown to reverse or ameliorate RTT-like symptoms in female mouse models. Therefore, conceptually, gene replacement is well suited as a strategy for this disease.