Rett syndrome is associated with MECP2 gene mutations, but the type of mutation, i.e. loss-of-function, gene duplication or triplication, and the degree of mosaicism for these mutations within cell types lead to highly heterogeneous phenotypic manifestations and clinical presentation ranging from microcephaly to normal brain size, shortened lifespan or not50. This evidence concerns the gene MECP2 and Rett syndrome.