<unk> s, also known as microsatellite expansion diseases, are a set of over 50 genetic disorders caused by trinucleotide<unk>  expansion, a kind of mutation in which<unk> s of three nucleotides (trinucleotide<unk> s) increase in copy numbers until they cross a threshold above which they become unstable. Depending on its location, the unstable trinucleotide<unk>  may cause defects in a protein encoded by a gene; change the regulation of gene expression; produce a toxic RNA, or lead to chromosome instability. In general, the larger the expansion the faster the onset of disease, and the more severe the disease becomes. Trinucleotide<unk> s are a subset of a larger class of unstable microsatellite<unk> s that occur throughout all genomes. The first trinucleotide<unk>  disease to be identified was fragile X syndrome, which has since been mapped to the long arm of the X chromosome. Patients carry from 230 to 4000 CGG<unk> s in the gene that causes fragile X syndrome, while unaffected individuals have up to 50<unk> s and carriers of the disease have 60 to 230<unk> s. The chromosomal instability resulting from this trinucleotide expansion presents clinically as intellectual disability, distinctive facial features, and macroorchidism in males. The second DNA-triplet<unk>  disease, fragile X-E syndrome, was also identified on the X chromosome, but was found to be the result of an expanded CCG<unk> . The discovery that trinucleotide<unk> s could expand during intergenerational transmission and could cause disease was the first evidence that not all disease-causing mutations are stably transmitted from parent to offspring. There are several known categories of<unk> . Category I includes Huntington's disease (HD) and the spinocerebellar ataxias. These are caused by a CAG<unk>  expansion in protein-coding portions, or exons, of specific genes. Category II expansions are also found in exons, and tend to be more phenotypically diverse with heterogeneous expansions that are generally small in magnitude. Category III includes fragile X syndrome, myotonic dystrophy, two of the spinocerebellar ataxias, juvenile myoclonic epilepsy, and Friedreich's ataxia. These diseases are characterized by typically much larger<unk>  expansions than the first two groups, and the<unk> s are located in introns rather than exons.
Trinucleotide repeat disorder
Lysosomal storage disease
Ribosomopathy
Lipid storage disorder
Trinucleotide repeat expansion
Creatine transporter defect
Cerebral creatine deficiency
Peroxisomal disorder