WILLIAM STUART, MD: MS is a very difficult disease to monitor. We expect progression, even under the best of circumstances. We know that none of the drugs are completely effective, so that you expect progression. If the patient is progressing, then it's hard to know whether it's driven by neutralization from the antibodies or that the drug that they're taking just isn't quite effective enough.

So it's a major impact for the patient and it's confusing for us and it's very hard sometimes to separate out these two issues, so you have to send off a specimen for antibody testing.

When considering the neutralizing antibody issue, we talk to our patients about and put it out to them as one of the issues that's involved in long-term treatment. It's important for them to understand these things, because while we don't know all of the answers in all of the issues relative to this particular problem, it's important that they know it and it's part of helping them factor in their decisions.