Chromosome studies in 10 patients with the Rett syndrome.

Chromosomes of white blood cells from 10 girls with the Rett syndrome, 9 of their mothers and 17 unrelated controls (5 girls and 12 women) were examined for the presence of the fragile site on Xp22 under different culture conditions. Six of the 10 Rett patients were fra(X)(p22) positive while 4 failed to express this fragile site. In the 6 patients expressing the fragile site it was present in 1 to 3% of all cells. Similarly, 5 of the mothers and 6 of the control women showed this fragile site, with a frequency of 1 to 9%. Concordance analysis showed that in the 9 mother-daughter pairs examined, 5 had concordant results while 4 of the pairs were discordant. These findings are very similar to those found in non-Rett individuals by ourselves and other investigators. These data agree with published findings that fra(X)(p22) is a common fragile site in normal individuals. Therefore, we conclude that the incidence of the Xp22 fragile site in the Rett syndrome does not differ from that found in various non-Rett individuals, including normal persons. This indicates to us that fra(X)(p22) cannot serve as a chromosomal marker for the Rett syndrome.