STRs database » FMR1 locus
Overview of repeats
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 (premutations) are associated with FXTAS (gene is overexpressed) whereas expansions over 200 repeats cause 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)). 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)).
For ovarian outcomes, Morbey et al. (2026) studied approximately 92000 women and found that 6.9% of 518 premutation carriers with available age-at-menopause data reported premature ovarian insufficiency. The authors described a continuous association with repeat length but limited individual prediction; ovarian insufficiency was self-reported in this study.