Methyl-CpG-binding protein 2 (MECP2) gene mutations have been identified in girls with Rett syndrome and in boys with heterogeneous neuropsychiatric disorders. Because of the limited or inconsistent data reported in literature, the role of methyl-CpG-binding protein 2 gene in the pathogenesis of mental retardation and pervasive developmental disorders needs further study. We scanned methyl-CpG-binding protein 2 gene in 99 Italian patients with pervasive developmental disorder or with nonsyndromal mental retardation. Four methyl-CpG-binding protein 2 gene mutations were found: 2 in 4 girls with Rett disorder, the others in 2 girls with mental retardation. The wide phenotypic spectrum and the variants of methyl-CpG-binding protein 2 gene, which may play an important role in gene regulation and neurodevelopment, justify the literature's interest particularly in girls.