Trinucleotide repeats in the FMR-1 gene hinder transcription

The FMR-1 gene in fragile X syndrome. (a) Exon structure and upstream CGG repeat. (b) Transcription and methylation in normal, premutation, and full mutation alleles. The red circles represent methyl groups.

[Data from W. T. O’Donnell and S. T. Warren, Annu. Rev. Neurosci. 25, 2002, 315-338, Fig. 1.]