Scientists are unraveling the mystery of what triggers Huntington’s disease, a devastating hereditary disorder that strikes in the prime of life. New research shows that a genetic mutation long linked to the fatal condition is harmless for decades before slowly turning into a toxic form that kills the cells. The cumulative death of those cells – not the mutation alone – eventually leads to problems with movement, thinking and behavior. The research was published Thursday in the journal Cell. One expert called it a “landmark study.”