Affected individuals have >200 CGG repeats and are classified as having a full mutation (FM) which is associated with hypermethylation of an upstream gene promoter region (CpG island) and silencing of FMR-1 gene
transcription which results in the absence of the gene product, fragile X mental retardation protein (FMRP).
Absence of expression of the FMR-1 gene
in fragile X syndrome.
Within the gene in question, the FMR-1 gene
on the X chromosome, about 30 repeats of a CGG triplet between the promoter and the start codon are normal.
The carrier frequency among women in the general population is estimated at 1 in 260, with the underlying cause a mutation in the FMR-1 gene
These premutations are unstable and can expand in size when passed from mother to child If the premutation expands to the size of a full mutation, the FMR-1 gene
"turns off" and does not work properly.
In these, the affected fmr-1 gene
had 57, 168, 182, 207, 266, or 285 repeats.