Around 90 % of the cases are explained by more than 800 reported mutations in the methyl CpG-binding protein 2 gene (MECP2) (RettBASE: MECP2 Variation Database) (Christodoulou et al. 2003), which is located in the X chromosome and which causes most of the classical or typical forms of RTT (Chahrour and Zoghbi 2007), and it was originally identified as encoding a protein that binds to methylated DNA (Lewis et al. 1992). The gene discussed is MECP2; the disease is Rett syndrome.