Fragile X Syndrome (trinucleotide repeat disorder)

• Genomic location of mutation: Xq27.3
• Type of mutation: trinucleotide repeat of CGG
• Normal range of the repeats: 6-50
• Intermediate number: 50-199 possess a premutation
• Fragile X repeats: >200 
• Diagnosis: screening with PCR followed by Southern blot
• PCR is not efficient with hight number of CGG repeats
• Prognoses: The large the number of the CGG repeats the earlier the onset of symptoms; more severe symptoms
• Mode of the mutations: the CGG repeats become methylated at the C nucleotide; methylation silences the gene
• a carrier is phenotypical normal in part because of the lack of FMR1 methylation