Interviewee: Matt Ridley. People with Huntington disease (HD) have an extended version of this HD or huntingtin gene. The extension is caused by a repeated region consisting of three bases: C-A-G. The larger gene produces a protein that accumulates over time and begins to kill brain cells when the person reaches middle age. (DNAi Location: Genome > Tour > Genome spots > Chromosome 4: Huntington disease > A chromosome 4 story)

Near the end of chromosome 4 lies the gene for Huntington, a protein necessary in the brain. When this gene has a particular mutation in the middle of it, the result is Huntington's disease. Huntington's disease is a terrible disease that strikes in late middle age, but people who have this mutation are fated from the moment of birth to get it. We now know exactly what the mutation is – we can predict at what age the disease will strike in different people. But at the moment we can do nothing about it. Diagnosis has run ahead of therapy.

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