The mutation in the gene is almost exclusively a new mutation (sometimes referred to as a De Novo mutation) occuring in that child  and the parents are not carriers of a mutated gene. In a very small number of cases,  more than one child with Rett syndrome has been born to the same parents. Where this occurs geneticists have found that there are  female family members who have a mutation of their MECP2 gene but do not show clinical symptoms.  These females are known as “asymptomatic female carriers. However, the risk of having a second child with the disorder is less than one percent.