Rett syndrome in a boy with a 47,XXY karyotype confirmed by a rare mutation in the MECP2 gene

Rett syndrome (RTT) is an X-linked condition which affects almost exclusive ly females. Here we report the first case of RTT syndrome in a boy with an XXY chromosomal constitution. Mutation analysis of the MECP2 gene in the af fected patient revealed a 423 C -->G substitution in exon 4, resulting in a new stop codon (Y141 X). This change was not present in both his parents o r in his older sister. Taking into account the incidence of both RTT syndro me as well as of Klinefelter syndrome, the probability for the simultaneous occurrence of these two events is very low (about similar to 1 in 10 to 15 ,000,000 births). However, the recent identification of mutations in the ME CP2 gene in affected males indicates that screening of the MECP2 gene shoul d be considered also in males with severe mental retardation (MR) in whom t he most common forms of MR have been excluded.