MECP2 and atypical Rett syndrome: Even though MECP2 is expressed ubiquitously [14], MECP2 mutations and copy number variations in humans lead to neurological phenotypes such as classic or atypical Rett syndrome and in rare cases Angelman Syndrome, X-linked mental retardation and Autism (reviewed in [15]) suggesting a distinct role for MeCP2 protein in the brain [16, 17].