It is not well understood what causes a trinucleotide repeat to expand into more than the usual number of copies that should be in a gene. Sometimes, a person may have more than the usual number of copies, but not enough to alter the function of the gene. These individuals are referred to as "permutation carriers." When they pass on these extra copies to a child, however, the extra trinucleotide repeats cause the DNA to become unstable, and the area of DNA expands even more. The result is that the child has a gene that no longer functions, or is not functioning properly and they are said to have the "full mutation." An example of a trinucleotide repeat disease is Fragile-X syndrome.

The gene for Fragile-X, called FMR-1, is located on the X chromosome. Females are usually not as severely affected as males because females have a normal X chromosome in addition to the X with the mutation. Also, females with the full syndrome are less common (about half the number of affected males) because they have two X chromosomes. The FMR-1 gene normally has 6 to 54 trinucleotide repeats. Premutation carriers have about 55 to 230 trinucleotide repeats, and persons with Fragile-X syndrome have over 230 repeats.