We have found no evidence of additional cases of PML.

Even under close evaluation, PML remains a rare disease. But as we alter the immune system, we need to understand what unintended effects it might have.

Statistically then, the risk is approximately 1 in 1,000, which is substantially higher than in the normal population. We're also careful to say we do not know what the risk is for individuals with long-term use of the drug.

We concluded that at this particular time there is no evidence that there are more PML cases.

We had to assess who else might be at risk.

This study was designed to address a critical, immediate need for patient safety in the people who participated in clinical trials of this drug.