Nature Neuroscience has a new weblog which promises to be more interactive and speculative than its companion peer-reviewed scientific print journal. One of the interesting early posts is about natalizumab. Seemingly the most effective drug ever developed against multiple sclerosis, the drug has been pulled from the market because of an indubitable but inexplicable link to the development of the devastating brain disease PML. For those of you interested in such things, the post discusses an intriguing hypothesis about how matalizumab might potentiate PML.