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

Due to the rapid enrollment of patients in the ... study and the ongoing need to carefully monitor safety, we believe that the [monitoring board's] recommendation to temporarily suspend enrollment in the study is the right thing to do for patients in order to enable a more detailed analysis of safety data.

We believe that the [safety monitors'] recommendation to temporarily suspend enrollment in the study is the right thing to do for patients in order to enable a more detailed analysis of safety data.

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 have found no evidence of additional cases of PML.