Fragile X syndrome is a genetic condition that affects how the brain develops, and it is the most common inherited cause of intellectual disability and one of the most common single-gene causes of autism spectrum disorder. It happens because of a change in a single gene called FMR1, which sits on the X chromosome – one of the two chromosomes that determine biological sex.

Inside the FMR1 gene is a short, repeating sequence of DNA letters (cytosine-guanine-guanine, written as CGG) that everyone has. Most people have somewhere between 5 and 44 repeats of this sequence, and that is considered typical. Problems begin when the number of repeats grows too large. Between 45 and 54 repeats is called an “intermediate” or “gray zone” range and usually does not cause symptoms, though it can occasionally grow further in future generations. Between 55 and 200 repeats is called a “premutation” – people who carry this range are usually not affected by Fragile X syndrome itself, but they can pass on a larger, more unstable repeat to their children, and they face their own separate health risks later in life. Above 200 repeats, the gene becomes chemically switched off, a process called methylation, and stops making a protein called FMRP that nerve cells need to communicate properly. This is the “full mutation,” and it is what causes Fragile X syndrome.
Because the mechanism involves the number of repeats growing across generations, doctors sometimes describe it as an “unstable” or “dynamic” mutation, distinct from most inherited conditions where the mutation itself does not change size from parent to child.