Thirty-three samples of peripheric blood lymphocytes, obtained from RTT girls and their parents, were analyzed by FISH with chromosome X-specific alphoid probe (pYAM10-40). In cells of eight RTT girls (24.2%) homol­ogous highly heteromorphic chromosomes X were found. In metaphase and interphase, one X chromosome had large or medium hybridization signals and the other chromo­some contained a very small hybridization signal (Fig. 1) This provided us with the possibility to accurately mark and differentiate the parental chromosome X. Chromo­somal preparations of an RTT girl’s parents were analyzed to determine the paternal or maternal origin of chromo­some X with alphoid DNA heteromorphism. Five girls had X chromosomes with small centromeric heteromorphism of paternal origin and three of maternal origin. The data of molecular-cytogenetic assays are shown in Table 1. Ac­cording to the molecular-cytogenetic study, one girl had random X-inactivation (55:45), four girls had moderate X-inactivation (from 63:37 to 74:26), and three girls had extremely skewed X-inactivation (more than 80:20).

We have performed a PCR-based methylation sensi­tive method for determining X-inactivation skewing on peripheric blood lymphocytes of the eight RTT girls. The results of HpaII digestion and PCR are presented in Fig. 2.

The data obtained and comparison of the two methods applied are presented in Table 1. There is good correlation of the results obtained by these two independent methods for X-inactivation analysis. Using molecucular-cyto­genetic and replication pattern analysis of active/inactive X chromosomes, we have found that active X chromo­somes have paternal origin in seven cases and maternal in one case of RTT girls (Table 1).