SMC1A-DEE is clearly distinct from SMC1A-related CdLS but resembles the clinical phenotype of Rett syndrome (RTT), a severe DEE mostly caused by heterozygous mutations of the X-linked MECP2 gene, which encodes a chromatin regulator to mediate DNA methylation and gene repression in the brain [9,25]. This evidence concerns the gene MECP2 and Cornelia de Lange syndrome.