The significance of X-inactivation for RTT clinical manifestations and inheritance was commonly reported [9,10,18]. There are a number of results concerning the frequency of cases with skewed X-inactivation in RTT. The frequency of RTT girls with skewed X-inactivation varies from 21% (DS >80%) [10] to 43% (DS >75%) [19], while in normal females, it can vary from 4 to 17% (DS >90%) and from 17 to 38% (DS >75%). These differ­ences could probably be explained by a different interpre­tation of the results rather that technical errors. Some au­thors consider a skewed X-inactivation of 90:10 or more, while other authors chose the levels of 80:20 or 75:25 as the data for skewed X-inactivation [10,19]. For example, our study of a Russian cohort of 28 RTT girls with MECP2 mutations, indicated an extremely skewed X-inactivation (90:10 or more) in only one girl [13]. How­ever, if the level of skewed X-inactivation was lowered to 80:20, six girls (or 21%) should be considered as having skewed X-inactivation [10]. It remains unclear if there is an increase of skewed X-inactivation frequency in RTT Thus, the study of skewed X-inactivation is of great inter­est for further investigations, due to its influence on RTT girl phenotypes.

It was shown that MECP2 mutations are almost exclu­sively of paternal origin [20]. Here, a prevalence of skew­ed X-inactivation in RTT girls with homologous highly heteromorphic X chromosomes and preferential inactiva­tion of the maternal chromosome, were detected by a molecular-cytogenetic approach, and confirmed by the PCR-based technique (seven out of eight girls). Thus, we propose that skewed X-inactivation against maternal (without mutation) chromosome X is a common feature of the classical form of RTT with sporadic incidence of MECP2 mutations.

We have shown that original chromosome-specific DNA probes allow identification of the paternal or mater­nal origin of active (inactive) chromosome X, based on homologous heteromorphism [9]. On comparing the results obtained by FISH and PCR, we found the FISH technique provided practically the same results as PCR. Therefore, molecular-cytogenetic assay using metaphase chromo­somes of individuals with heteromorphic chromosome X could be applied for X-inactivation analysis. This ap­proach could probably be useful for interphase molecular-cytogenetic analysis.