MEF2C and atypical Rett syndrome: Interestingly, either too much or too little transcriptional activity due to mutation or changes in electrical activity (e.g., affecting MEF2C or MeCP2, a chromatin-associated protein that can either activate or repress transcription) can lead to dysfunction, particularly during development, with decreased MEF2C activity (MEF2C haploinsufficiency syndrome) or MeCP2 activity (Rett syndrome) resulting in severe cases of autism spectrum disorder/intellectual disability (Li H. et al., 2008; Lipton et al., 2009; Chahrour and Zoghbi, 2007).