The first drug that can potentially correct the underlying defect that causes Huntington's disease has been taken by patients in a clinical trial.
Doctors at University College London, which is leading the study, said it was an important moment in tackling the incurable condition.
Current medication treats the symptoms, but cannot slow or prevent the progressive damage to the brain.
The Huntington's Disease Association said the trial was "very exciting".
The disease is caused by the brain producing a mutant protein called huntingtin which damages and ultimately kills off brain cells.
As Huntington's progresses it leads to uncontrolled movements, behaviour changes and poor cognition. Life expectancy after diagnosis can be as short as 10 years.
The drug, known as ISIS-HTT, is from an experimental class of medicines known as "gene silencers".
The huntingtin gene in a patient's DNA contains the instructions for building the destructive protein.
Those blueprints are carried to a cell's protein-making factories and the drug effectively kills the messenger.
The trial will be led by Prof Sarah Tabrizi, the director of the Huntington's Disease Centre at University College London.
She told the BBC News website: "It's the beginning of quite an important journey in Huntington's disease, it is clearly very early but this is a step forward.
"The preclinical work shows that if you lower production of the mutant protein then animals recover a large amount of motor function.
"Huntington's is a really terrible disease that blights families. I know a mother whose husband and three children were affected, this would have a massive impact [if it works]."
The trial will test the drug's safety by progressively increasing the dose in 32 patients.
It will be injected into the spinal cord of patients once a month for four months and they will then be observed for a further three months.
Clinicians will be ensuring there are no dangerous side-effects, such as allergic reactions, as well as measuring the impact on levels of the corrupted huntingtin protein.
At the highest doses they hope to halve levels of the protein.
Cath Stanley, the chief executive of the Huntington's Disease Association, told the BBC News website: "There's a lot of different trials and avenues of research, but this is the most exciting.
"People develop Huntington's disease between the age of 30 and 50 so delaying it for a few years allows people to spend more time with family in the prime of their life.
"This is the first, potential, major breakthrough in terms of delaying symptoms of Huntington's disease, it's such an exciting step forward."
The drug has been developed by ISIS-pharmaceuticals.
It targets strands of genetic code called messenger RNA which carry instructions for huntingtin out of a cell's nucleus.
The drug is a manufactured stretch of genetic code that is the mirror-image of the messenger RNA that binds strongly to it to neutralising it.
In the UK, 12 in every 100,000 people have the condition.
Follow James on Twitter.