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What happened

bluebird bio (BLUE 7.96%) closed down 13% Friday after presenting an update on its LentiGlobin programs and other research and development projects at its Gene Therapy Day yesterday.

So what

R&D days are supposed to be a place for biotechs to showcase what they're working on, and they usually don't have a negative effect on the company's stock price. In fact, Bluebird's shares closed flat yesterday.

It appears today's decline comes from investors selling the news. Shares had run up in anticipation of the data release, so it shouldn't be much of a surprise that traders are selling now that there isn't another near-term catalyst.

At least one analyst, Elemer Piros from Cantor Fitzgerald, downgraded Bluebird, arguing that the upcoming American Society of Hematology (ASH) meeting in December will be a non-event.

Technically, Piros is right. Bluebird changed the manufacturing process for its LentiGlobin gene therapy, increasing the number of cells that took up the gene to be expressed by threefold. More cells expressing LentiGlobin should result in more patients being cured of their transfusion-dependent beta-thalassemia and severe sickle cell disease, which the gene therapy treats.

The switch to the new manufacturing process happened recently, so the data at ASH comes from the old process. But that was known well before Bluebird's R&D day yesterday, so it doesn't seem like a reason to sell.

Now what

You could argue that Bluebird is in a no-lose situation going into ASH. All the old manufacturing process needs to do is produce decent -- but not necessarily outstanding -- results and investors should be convinced that the new process will likely put it over the edge. If the results are disappointing, Bluebird can argue that the new manufacturing process is so much better than the old one that the disappointing results aren't relevant.