Trinucleotide repeats are sets of three nucleotides present in succession in various copy numbers throughout the human genome. These areas of the genome are unstable and polymorphic. Trinucleotide repeat disorders are a series of conditions caused by triplets in a mutated gene that are present in greater number than found in normal genetic sequencing. These abnormal sequences are known as "expansions" and present across many stages of human development and numerous cell types. Repetitive sequences of genetic code are quite common. However, when these sequences grow beyond the scope of what would be considered normal, they cause disease. While the human genome has mechanisms to protect against these expansions, patients present with what can be severe neuromuscular and neurodegenerative disorders. There have been many diseases discovered by TNR expansions, but the most prominent are spinocerebellar ataxia, Huntington disease, Fragile X syndrome, myotonic dystrophy, and Friedrich ataxia. These disorders, as well as the genetic and molecular development of these disorders, will be discussed below.