Studies have implicated de novo mutations of the X-linked methyl-CpG-binding protein 2 (MECP2) gene (OMIM∗300005) in the majority of the RTT cases, while mutations in cyclin-dependent kinase-like 5 (CDKL5) and forkhead box G1 (FOXG1) have been more rarely reported [3–5]. This evidence concerns the gene CDKL5 and Rett syndrome.