Definition: Refers to a sequence of three DNA building blocks (nucleotides) that is repeated a number of times in a row. DNA segments with an abnormal number of these repeats are unstable and prone to errors during cell division. The number of repeats can change as the gene is passed from parent to child. If the number of repeats increases, it is known as a trinucleotide repeat expansion. In some cases, the trinucleotide repeat may expand until the gene stops functioning normally. This expansion causes the features of some disorders to become more severe with each successive generation. When the cause of a disease can be traced to having too many copies of a certain trinucleotide repeat, the disease is said to be a trinucleotide repeat disorder. An example of a trinucleotide repeat disorder is Huntington disease.