In fragile X syndrome, what is the number of repeats for a full mutation?

In fragile X syndrome, a full mutation is characterized by an expansion of the CGG triplet repeats within the FMR1 gene to a range of 200 to over 2,000 repeats. This significant increase in the number of repeats leads to the methylation of the promoter region of the gene, resulting in silencing and preventing the production of the fragile X mental retardation protein (FMRP). The lack of FMRP is associated with the cognitive and developmental challenges seen in individuals with fragile X syndrome.

Other values outside this range, such as 5-55, 56-200, and 9-37 repeats, correspond to intermediate or pre-mutation statuses. In a normal individual, the number of CGG repeats is typically between 5 to 44, and a premutation status, which may result in potential transmission and risk to offspring, falls between 55 and 200 repeats. However, these do not result in the full manifestation of fragile X syndrome. Hence, the definition of a full mutation as 200 to 2,000+ repeats is critical for understanding both the molecular basis of the syndrome and its clinical implications.