Are there different kinds of FMR1 mutations?

Yes. An individual can have the following:

1. Normal: Individuals with less than 45 CGG repeats have a normal FMR1 gene.
2. Intermediate: Those with 45-54 CGG repeats have what is called an “intermediate” or “gray zone allele,” which does not cause any of the known Fragile X associated disorders.
3. Premutation: Individuals with 55-200 CGG repeats have a premutation, which means they carry an unstable mutation of the gene that can expand in future generations and thus cause Fragile X syndrome in their children or grandchildren. Individuals with a premutation can also develop FXTAS or FXPOI themselves.
4. Full Mutation: Individuals with over 200 CGG repeats have a full mutation of the FMR1 gene, which causes Fragile X syndrome.

The full mutation causes the FMR1 gene to shut down or “methylate” in one region. Methylation happens to other genes too, when they are supposed to be turned off (as in the genes we don’t use, like those that make a tail grow!).

Learn more:

• Understanding a Fragile X Intermediate Result
• FMR1 Gray Zone Allele: What Do We Know About It?
• Premutation Carriers
• Understanding Fragile X syndrome 
• Genetics & Inheritance
• 7 Things You Didn't Learn About Fragile X in Biology Class