Our DNA contains four types of molecules, adenine, cytosine, thymine, and guanine—called nucleotides or bases—running along the strands that make up our chromosomes. Since there are only four, it is not unusual to find a short sequence of DNA repeating itself somewhere in the genome. However, when a sequence is repeated too many times in a row—dozens or hundreds of times—it can cause problems for cells. Repeat expansion disorders are a category of neurological disorders driven by these excessive repeats, and they can cause neurodegeneration, muscular dystrophy, intellectual disability, and more.