True about fragile-X syndrome

Triplet Repeat Mutations: Fragile X Syndrom Fragile X syndrome is the prototype of diseases in which the causative mutation occurs in a long repeating sequence of three nucleotides. Other examples of diseases associated with trinucleotide repeat mutations are Huntington disease and myotonic dystrophy. About 40 diseases are now known to be caused by this type of mutation, and all disorders discovered so far are associated with neurodegenerative changes. (Robbins Basic Pathology,9th edition,pg no. 241)