MECP2 is located in Xq28 chromosome region, it is composed of four exons which encode the Methyl-CpG-binding protein2 , 9 ). This protein is involved in neuronal development and maturation, as well as in the differentiation and formation of neuronal synap ses, by regulating gene expression through CpG island methylation, acting both as a repressor and trans criptional activator10,11,12. The protein has four functio nal domains, of which the most important are MBD (Methyl-CpG-binding Domain) and TRD (Transcrip tional Repression Domain)1 , 11 ).In Chile, the molecular bases of Rett syndrome has not been characterized. In this study, the type and fre quency of variants in MECP2 in Chilean patients with a clinical diagnosis of RTT were analyzed.