Skewed X chromosome inactivation is considered a common feature in families with X-linked mental retarda­tion. Rett syndrome (RTT) has particular significance among this group of diseases. A widely used technique for determining of skewed X chromosome inactivation is the polymerase chain reaction (PCR)-based method for deter­mining methylation patterns of the human androgen recep­tor gene. Recently, a molecular-cytogenetic X-inactiva­tion assay, based on step-wise application of differential replication staining and fluorescence in situ hybridization (FISH) techniques to identify the inactivation status of paternal and maternal chromosome X in RTT girls, was described. The use of FISH, with an original alphoid chro­mosome X-specific DNA probe, allows the differential marking of parental X chromosomes with reference of their inactivation status (active or inactive). The compara­tive analysis of these two techniques for molecular-cyto­genetic assay development, allowing analysis of X-inacti­vation in interphase and metaphase cells, was carried out. A prevalence of skewed X-inactivation in RTT girls and preferential inactivation of maternal chromosomes were detected. Correlation of the results obtained by these two independent methods for X-inactivation analysis was done.

Comparing the results obtained by FISH and PCR, we have found the FISH technique providing practically the same results as PCR. The perspectives of molecular-cyto­genetic X-inactivation assay application are discussed

Key words: Chromosome X inactivation; Fluores­cence in situ hybridization (FISH); Mental retardation, Rett syndrome (RTT).