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 of biopharmaceutical company Celgene (Nasdaq: CELG) fell as much as 13% following updated guidance on plans for its blockbuster drug Revlimid, and news from a competitor.

So what: Celgene is being hit on two fronts today. First, Revlimid, which is a cancer therapy used to treat multiple myeloma, a bone-marrow cancer, and accounted for $861 million of Celgene's $1.25 billion in sales in the first quarter, won't be expanding its reach nearly as quickly as the company and shareholders had hoped. Celgene withdrew its application in the European Union for newly diagnosed cases of multiple myeloma and pushed back its plans to gain additional marketing therapies approved in the U.S. until 2013.

In addition, Onyx Pharmaceuticals' (Nasdaq: ONXX) multiple myeloma drug, carfilzomib, received a favorable ruling from the Food and Drug Administration's advisory panel despite previous concerns about its safety risks. Again, this doesn't mean a guaranteed approval, but it usually serves as a good indication that the drug eventually may be approved. Carfilzomib would be a treatment that would follow patients who have tried Velcade from Takeda Pharmaceuticals and either Revlimid or Thalomid from Celgene.

Now what: Although Celgene's growth plans for its largest drug have been halted, today could be an attractive buying opportunity. Celgene did confirm its earnings guidance despite the new drug application delays and still expects sales to rise to $8 billion-$9 billion by 2015 with EPS ranging from $8-$9. I know we're looking way down the tunnel here, but that'd be a forward P/E of about seven with sales growth of at least 60% from fiscal 2011 to fiscal 2015. I can't say I like Celgene's reliance on one drug for the majority of its sales, but further indications will extend its patent protection and ease investor worries.

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