Hereditary unstable DNA is composed of strings of trinucleotide repeats, in which three nucleotides are repeated over and over (ie CAGCAGCAGCAG). These repeats are found in several sites within genes; depending on their location, the number of triplet repeats in a string can change as it is passed on to offspring. When the number of repeats increases to a critical size, it can have a variety of affects on gene function. The repeats may cause a loss in gene function (as in Fragile X) or may result in the gain of a new, abnormal protein and thus a new function (as in myotonic dystrophy and Huntington disease). Although a variety of trinucleotide repeat diseases have been reported and merit consideration, this discussion will focus primarily on Fragile X syndrome, myotonic dystrophy, and Huntington disease.