Chromosome findings in the Rett syndrome and a test of a two-step mutation theory.

Chromosome findings are described in 22 girls with the Rett syndrome. One of the girls had a duplication of chromosome 6, another showed a deletion X(p22----pter) in 10% of the cells from fibroblast cultures. Fragile X(p22) was seen in 9 of the 22 girls with Rett syndrome. A relationship between the disorder and fragile X(p22) could not be proven. Family data contradict our theory of a two-step mutation as the cause of the Rett syndrome.