|Number of repeats||Counts (alleles)||Percentage|
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).