The defect that causes the neurodegenerative disease Huntington's has been corrected in patients for the first time, the BBC has learned.
An experimental drug, injected into spinal fluid, safely lowered levels of toxic proteins in the brain.
The research team, at University College London, say there is now hope the deadly disease can be stopped.
Experts say it could be the biggest breakthrough in neurodegenerative diseases for 50 years.
Huntington's is one of the most devastating diseases.