MECP2 and Rett syndrome: Today, several mutations in the X-linked MECP2 gene are identified, being acknowledged as the cause of 95% of the classical RTT cases and of 40–50% of the atypical RTT cases (Neul et al., 2010; Zhang et al., 2017), resulting in a wide genetic and phenotypic heterogeneity of this disease (De Felice et al., 2012).