The Rett syndrome (RTT; OMIM #312750) is a rare genetic disease observed predominantly among girls that affects neurological development. The incidence of this disorder is approximately 1 in 10,000 female births. Diagnosis of the RTT is based on specific clinical criteria and the identification of a mutation in the methyl-CpG-binding protein (MECP), which mainly occurs on exons 3 and 4 of the gene. Mutations in the X-linked methyl-CpG binding protein 2 gene (MECP2) are observed in nearly 95% of RTT cases. RTT is associated with considerable genotypic and phenotypic heterogeneity. Recently, it has been observed that mutations in the genes Netrin G1 and cyclin-dependent kinase like 5 (CDKL5) also lead to clinical pictures resembling RTT. In this case report, we describe a 4-years-old female patient who met all the relevant criteria for the diagnosis of RTT. Sequence analyses performed on the patient identified a de novo, heterozygous c.489G>A mutation at exon 4 of the MECP2 gene.