MECP2 and Rett syndrome: Around 90% of the patients are diagnosed with a point mutation or small deletions in the MeCP2 gene (http://mecp2.chw.edu.au/, http://www.biobank.unisi.it, http://www.MeCP2.org.uk/ (accessed at 10 March 2021)), causing the symptomatic heterogeneity observed in classical RTT patients.