A small subset of atypical cases with a RTT-like phenotype, characterized by the early onset of seizures, is associated with mutations in CDKL5 (Cyclin-Dependent Kinase-like 5) [10-12], whilst some individuals with the congenital RTT variant have mutations in the FOXG1 (forkhead box protein G1) gene [13], and still others remain genetically undefined. Here, CDKL5 is linked to Rett syndrome.