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How the length of the FMR1 gene expands to cause fragile X syndrome


  • Within the FMR1 gene is a repeated DNA sequence ‘CGG’. This sequence is normally repeated between 6 and 44 times. This number of repeats is almost always stable and someone with 6-44 repeats in their FMR1 gene will pass on the same number of repeats to their children.
  • Larger repeat sizes can be unstable and there is a chance that the number of repeats will expand when passed from mother to child but not father to child.
  • Women with 45-54 repeats (grey zone) will usually pass on the same number of repeats to their children, but there is a chance the number of repeats will increase to a medium length gene (50-200 repeats) and the child will be a carrier of fragile X syndrome.
  • For women with a medium length gene there is a chance that the number of repeats will expand to over 200 repeats when it is passed on and the child will be affected by fragile X syndrome.

Repeat length Gene status Effect
6-44 Short Normal
45-54 Intermediate Grey zone
55-200 Medium Carrier of FXS
>200 Long Affected with FXS