Interestingly, in a rare case of the male proband aged 2.5 years with Rett syndrome, we observed that the variant c.538 C > T (p.Arg180Ter) in the MECP2 gene originated through a post-zygotic de novo event which led to somatic mosaicism in the proband (Table 3) [49]. This evidence concerns the gene MECP2 and Rett syndrome.