premutation is seen in:

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 In the normal population, the number of repeats of the sequence CGG in the FMR1 gene is small, averaging around 29, whereas affected persons (Fragile X syndrome) have 200 to 4000 repeats. These so-called full mutations are believed to arise through an intermediate stage of premutations characterized by 52 to 200 CGG repeats. Carrier males and females have premutations. (Robbins Basic Pathology,9th edition,pg no. 241)