Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares in Biogen (BIIB 0.51%) tumbled as much as 11% today after the company reported that a patient that had been taking the company's fast-growing multibillion blockbuster drug Tecfidera died after being diagnosed with progressive multifocal leukoencephalopathy, or PML, a rare viral disease that damages the material that protects white matter of the brain.

So what: Tecfidera is the top selling oral MS drug for reducing multiple sclerosis relapses. Tecfidera's sales have skyrocketed 316% year over year to nearly $2 billion through the first nine months of this year and sales have been expected to broadly trend higher from here given Biogen is still rolling it out overseas. Although PML is a rare disease, it's still a dangerous disease with a high mortality rate of between 30% and 50%. Given that backdrop, investors are worrying that more cases of PML may be identified, causing sales of Tecfidera to slump as oral MS drug competitors Novartis (NVS -1.02%), which markets Gilenya, and Sanofi (SNY 0.52%), which markets Aubagio, regain market share.

Source: Biogen and Author's Calculations

Now what: Similar concerns were raised for Novartis' last fall when a Gilenya patient was diagnosed with a case of PML.  A review of the patient's MRI resulted in Novartis' questioning whether the patient had PML prior to being dosed with Gilenya and despite questions remaining, Gilenya sales are still running at a $2.4 billion annualized clip this year, up from about $2 billion in 2013.

This isn't the first time a Biogen MS drug has been associated with PML, either. In 2005, Biogen's Tysabri, an injection-based selective monoclonal antibody drug that inhibits a key step in the cause of multiple sclerosis, was temporarily removed from the market when three Tysabri patients were diagnosed with PML. Tysabri's case led to an examination of 3,000 Tysabri patients during which it was determined that the likelihood of a Tysabri patient being diagnosed with PML was just 0.10%. As a result, Tysabri was reintroduced in 2006 and it's gone on to become a blockbuster drug with sales totaling $1.475 billion through the first nine months of 2014.

Given MS doctors already have PML protocols in place for other MS therapies, the risk to Tecfidera's market share may prove to be less than some investors may be thinking. Regardless, Biogen investors will want to watch this situation carefully over the coming quarters to see whether script volume slides, or more PML cases are diagnosed.