For Prue Willday, whose father died of the disorder when she was 11, the discovery means far more than a scientific breakthrough. 'When I was told at the weekend I felt this enormous lift. Thank God something's happening.'
Mrs Willday, who is 40, has known for most of her life that she has a 50:50 chance of having inherited the defective Huntington's gene from her father. If she has the gene, she faces the prospect of a lingering death in middle age.
Scientists are now confident they will soon develop an accurate genetic test for all people at risk of Huntington's. Up to now a test has existed for some at-risk families but it has not been 100 per cent accurate. The chance of an error is one reason why Mrs Willday elected not to take it. If she had, the feeling that the result could be wrong would have put her under additional pressure.
But even an accurate test will fail to tell her when she will succumb to the first symptoms of nervous degeneration - mental impairment and psychiatric disturbance - that many people at risk find themselves looking out for in mid-life. Her sister died from Huntington's disease at 34; her aunt at 60. 'It would be easier in some respects to know when. If you knew that in 10 years' time you'd develop the disease, it means you've got 10 years without having to watch yourself.'
Another reason for not taking the test is, of course, the lack of a cure, or any effective treatment. Taking the test and finding it positive will mean the certainty of a slow, premature death. Even a negative result, showing she has not inherited the disease, could pose problems, she says. 'It has been a prop for my life. Perhaps deep down there is a feeling that people should be more tolerant of me because of this sword of Damocles hanging over me.'
If Mrs Willday took the new test, it would have implications for her 14- year-old son and 11-year-old daughter. Both know they have a one in four chance of having inherited the gene from their maternal grandfather; if Mrs Willday proved positive, this risk would jump to 50:50.
Perhaps the hardest dilemma is if her children want to take the test when they are 18 (present rules forbid the testing of children), while their mother continues to refuse it. The difficulty, she says, is that a positive result for one child would mean a positive result for her. 'The ethical problem is their right to know and my right not to.'
Mrs Willday, a solicitor who is helping to draft an international protocol on this subject, is better placed than many to cope. She has a family that discusses the problems rather than denying they exist. Others are not so fortunate.