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What: Shares of biotech blue chip stock Biogen (NASDAQ:BIIB) were creamed in April, with its shares dipping 11% according to data from S&P Capital IQ, after the company reported disappointing first-quarter earnings results on April 24.

So what: For the quarter, Biogen delivered $2.56 billion in revenue, a 20% increase over what it delivered on the top line in the year-ago period. This was led by Biogen's trio of multiple sclerosis giants, including $825 million from oral relapse-remitting drug Tecfidera, $693 million from Avonex, and $463 million from Tysabri. New MS drug Plegridy contributed $62 million, while Biogen's hemophilia duo nearly delivered $97 million combined.

The problem, put plainly, was weakness in Tecfidera. Sales of the drug were about $110 million below what Wall Street had been expecting, and ultimately this translated into an adjusted profit of $3.82 per share, also $0.09 light of consensus estimates. Biogen blamed Tecfidera's weakness on one fewer shipping week in the U.S. and increased discounts on the drug. Excuses weren't what investors were looking for from a stock that's up more than 665% over the past five years.

Now what: What we're seeing here is one of the few downsides to owning Biogen: its overwhelming reliance on a single disease (MS) to drive its growth. While I don't doubt that Biogen has a product lineup that can sustain profits for years to come, investors also have to realize that Tecfidera is nearing its peak sales saturation point, which is likely around $3.5 billion to $4 billion. For investors it means Biogen's top-line growth is going to slow unless its hemophilia drugs and Plegridy really ramp up quickly.

Source: Biogen.

Biogen also has a lot riding on its two experimental compounds, aducanumab for Alzheimer's disease, and anti-Lingo-1 for MS as a next-generation drug. Both have shown promise thus far (aducanumab more so in my view), but both experimental drugs are also in fields with a very high rate of failure. There have been a handful of Alzheimer's drugs that have wowed in phase 1 studies only to fail miserably in late-stage studies, which is where aducanumab is now. There's quite the premium built into Biogen's share price based on the early results from aducanumab and anti-Lingo-1, so additional positive or negative data will almost certainly sway Biogen's share price.

As for me, I continue to view Biogen as the equivalent of a hold, or a wait-and-see. I believe long-term investors will do just fine holding on or even dipping their toes into the water, but its lack of therapeutic diversification is a bit worrisome and leads me to believe there are faster growing and cheaper biotech blue chip alternatives out there.