MECP2 and atypical Rett syndrome: Rett syndrome (RTT) is one of the most common causes of intellectual disability and predominantly results from loss-of-function mutations within the MECP2 gene, located on the X chromosome, which encodes methyl-CpG binding protein 2 (MeCP2).72,73 RTT is estimated to affect one in every 10 000 to 15 000 females and is hardly ever diagnosed in males.74,75 In humans, RTT is characterized by developmental stagnation and regression, starting between 6 and 18 months of age.