Editor—Rett syndrome (RTT) (MIM 312750) is an X linked dominant neurodevelopmental disorder that occurs almost exclusively in females. Affected girls are considered to have a normal perinatal period followed by a period of regression, loss of acquired purposeful manual and speech skills, hand wringing, gait disturbance, and growth retardation.1 2

A gene for RTT has been identified in the Xq28 region which encodes the methyl-CpG binding protein 2 (MeCP2) involved in transcriptional silencing.3 4 This disorder most frequently occurs sporadically and results from a de novo mutation, although a few familial cases have been reported. Many studies5-16 have shown that the MECP2 gene is mutated in approximately 80% of patients with classical RTT and theMECP2 mutation spectrum includes missense, nonsense, and frameshift mutations, as well as larger rearrangements like deletions encompassing a few hundred bp.16 The failure to detect MECP2 mutations in the remaining 20% may indicate the presence of mutations in unexplored regions of the MECP2 gene, such as regulatory elements or non-coding regions, notably in the new first exon17 or in an additional RTT locus.

Here, we report for the first time mosaicism for a somaticMECP2 mutation found in two unrelated females affected with RTT. These two girls were diagnosed according to the international criteria of the Rett Syndrome Diagnostic Criteria Work Group.18