Because the MECP2 mutation is the most commonly observed cause of the Rett syndrome, the disease caused by the MECP2 mutation is called the typical Rett syndrome, whereas that involving the other genes is historically called the atypical Rett syndrome (CDKL5 mutation, Hanefeld variant; FOXG1 mutation, congenital variant) [9–11]. Here, MECP2 is linked to Rett syndrome.