STRs database » FMR1 locus
Gene FMR1
Location 5′ UTR in Xq27.3
Reference region chrX:147912050-147912110 (hg38)
chrX:146993568-146993628 (hg19)
chrX:146176678-146176768 (hs1)
Repeat unit CGG
Repeat type Standard
Distribution of repeats in a superpopulation
Overview of repeats

Both, Fragile X syndrome (FXS) and fragile X-associated tremor/ataxia syndrome (FXTAS) are related but clinically distinct diseases that are caused by CGG repeat expansion in the FMR1 gene (Mishra et al., 2019). In a healthy population, the repeat length in FMR1 is between 6 and 40 repeats (Aziz et al., 2003). Repeat expansions in the range 55-200 (pre-mutations) causes FXTAS (gene is overexpressed) whereas repeats over 200 repeats causes FXS (gene is silenced) (Mishra et al., 2019).
Additionally, an intermediate repeat range (so called grey zone) between 41-54 repeats has been defined (Aziz et al., 2003). One another study defined the range as 35-54 because the number of alleles in that range were significantly increased in patients with premature ovarian failure (Bretherick et al., 2005).

→ View STRipy's population-wide repeat data for FMR1 locus

Disease(s) and range of repeats
Disease Onset Inheritance Normal Intermediate Pathogenic
Fragile X tremor/ataxia syndrome (FXTAS) Adult X-linked dominant 6-40 41-54 55-200
Fragile X Syndrome (FXS) Paediatric X-linked dominant 6-40 41-54 ≥201