Story : Arlo, Nancy and Janice each have a 50/50 chance of developing a devastating nerve disorder. A laboratory test can tell them if in fact they will fall victim. In their shoes, would you take the test? Thousands of others face a similar choice: to know, or not know, if they will carry the genetic time bomb of Huntington's disease. NOVA looks at this incurable disease which affects 20,000 people in the US and threatens tens of thousands of others.