Huntington's disease (HD) is unique among the other neurodegenerative diseases in that it is always inherited. HD is caused by abnormal huntingtin protein. The normal huntingtin protein contains multiple repeats of a sequence of three DNA bases, cytosine-adenine-guanine (CAG), which encodes the amino acid glutamine. When more than 36 repeats are present in the protein, stemming from a mutation in the huntingtin gene, an abnormal protein is produced. This mutant form of the protein is more prone to aggregation than versions of the protein with a normal number of repeats. Individuals with 36–40 CAG repeats may or may not develop symptoms of HD, while people with more than 40 repeats will develop the disorder during a normal lifetime. No case of HD has been diagnosed with a CAG repeat count fewer than 36.