so all muscle then becomes deer in separated, unplugd from motor nerves, breathing fails and they die. >> rose: do we know what causes that neuron to die? >> so we don't. and, in fact, i'll comment on that because that's a burning question in terms of the way we understand this problem clinically. we suspect there are many factors that contribute. certainly about 10% of cases we know that there are mutant genes that make sick or toxic proteins that actually actively kill a neuron. but we also know that behavior probably plays a role. there's some data that soccer players in italy, for example, have a higher-than-expected incident of a.l.s. we know head trauma may be a factor. we know that the environment may also influences, of course, the disease. and there may be just some role for what we call bad luck. >> this is such an important principle because what bob is showing here for amy trow if i can lateral sclerosis is almost the general principle in the nervous system. most diseases-- parkinson's-- they have multiple causes. so one doesn't think of simply a single factor like genes.