The scientists examined the risk of developing the disease among people who were adopted into families in which one or more members developed MS. They found the adopted relatives had no increased risk over that of the general population, where the rate is one in a thousand.

Earlier studies had shown that natural-born children or siblings of MS patients have a 20 to 40 times greater risk. But until now it has not been clear whether the reason is genetic, or some environmental factor to which family members had all been exposed.

``Basically what we've shown is that the increased risk that occurs in families, which is quite substantial, is entirely determined by genes,'' Dr. George Ebers, professor of clinical neurological sciences at the University of Western Ontario, told The Medical Post.