Neither uniparental disomy nor skewed X-inactivation explains Rett syndrome.

The locus DXS255 was studied using the probe M27 beta in ten probands with Rett syndrome and in eight of their families. No evidence of uniparental disomy of the X chromosome was detected, as all informative probands had inherited an allele from each of their parents. Differential methylation of a CCGG site within the DXS255 locus as shown by digestion with MspI/HpaII, revealed moderate skewing of X-inactivation favouring the maternal allele in two of the probands. Random X-inactivation was present in all mothers tested and in two unaffected sisters. Three of four unaffected siblings had inherited the same maternal allele at DXS255.