What range of repeat sizes in the FMR1 gene indicates a full mutation associated with Fragile X syndrome?

The range of repeat sizes in the FMR1 gene that indicates a full mutation associated with Fragile X syndrome is more than 200 repeats. In this context, Fragile X syndrome is caused by an expansion of the CGG repeat in the FMR1 gene.

Normal individuals typically have between 5 to 44 repeats of the CGG sequence, while individuals with premutations have between 55 to 200 repeats. However, when the number of repeats exceeds 200, the gene becomes fully mutated, leading to the silencing of the FMR1 gene and resulting in the symptoms associated with Fragile X syndrome. This dramatic expansion prevents the normal expression of the protein produced by the FMR1 gene, which is essential for proper brain functioning, particularly in synaptic plasticity and cognitive development. Therefore, understanding these thresholds is crucial for diagnosing and managing Fragile X syndrome.