Rett syndrome (RTT) [Online Mendelian Inheritance in Man (OMIM) #312750] is a monogenic, severe disorder due to a genetic mutation of the Methyl-CpG-binding protein-2 (MECP2) gene that affects brain development with an incidence of 1 girl in 10,000–15,000 live births1. This evidence concerns the gene MECP2 and atypical Rett syndrome.