Table 2 Association between CGG repeat lengths, category, and clinical significance.

From: Laboratory testing for fragile X, 2021 revision: a technical standard of the American College of Medical Genetics and Genomics (ACMG)

# CGG repeats

Category

Clinical significance

≤44

Normal

No disease association. Rare cases with FMR1 deletion or base change may cause fragile X. Very low risk of CGG repeat expansion to next generation.

45–54

Intermediate or gray zone

No disease association. Very low risk of CGG expansion to full mutation within one generation.

55–200

Premutation

Males and females: fragile X–associated tremor ataxia syndrome (FXTAS). Females only: fragile X–associated primary ovarian insufficiency (FXPOI). Risk of expansion to full mutation in the next generation when maternally inherited.

>200

Full mutation

Fragile X syndrome: clinical severity influenced by sex, degree of methylation, and level of mosaicism.

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