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Updated from 1:10 pm EDT with new information.

Bluebird Bio (BLUE) - Get bluebird bio, Inc. Report shares fell 6.5% to $79.99 Monday afternoon -- the stock's lowest price this year -- after the company said a patient treated with a first-generation version of its gene therapy suffered a relapse seven years later.

The patient with beta-thalassemia, a rare blood disorder, required two blood transfusions because of clinical symptoms of anemia, Bluebird said. This patient was treated in 2007 with a precursor version of the company's gene therapy which is no longer used. All of Bluebird's ongoing clinical trials in beta-thalassemia and sickle cell disease use an improved, more potent gene therapy, the company said.

Still, Monday's long-term patient update -- made at a scientific meeting of beta-thalassemia experts -- was enough to spook investors worried the curative benefit of gene therapy may wane over time.

In an interview, Bluebird Senior Vice President of Clinical Development Robert Ross said the beta-thalassemia patient described in Monday's presentation was "always on the border of needing blood transfusion" even after being treated with the gene therapy, so it's not surprising, necessarily, that the patient's anemia worsened to a point seven years later where transfusions were required. 

The first-generation version of Bluebird's gene therapy, referred to as HPV569, was not capable of producing large quantities of functional hemoglobin compared to the current gene therapy, which Bluebird calls BB305.

To illustrate this point, Ross provided the following chart which compares the ability of the old and new gene therapies to produce HBA-T87Q, or functional hemoglobin, derived from the gene therapies. 

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The blue and green bars represent beta-thalassemia patients treated with HPV569, the company's precursor gene therapy. [The patient discussed in Monday's presentation is the blue bar.] The pink and magenta bars represent beta-thalassemia patients treated with the current Bluebird gene therapy in its clinical trials. Beta-thalassemia patient benefit more with increased functional hemoglobin (taller bars) derived from Bluebird's gene therapy.

"There is a remarkable difference in terms of the quantity of protein expression," said Ross. He cautions that these comparative data don't guarantee Bluebird's current gene therapy will be able to produce durable cures for beta-thalassemia and sickle cell patients, but it does show the relative lack of potency from the older version.

J.P. Morgan analyst Cory Kasimov defended Bluebird in a note to shareholders Monday afternoon. "We see the sharp drop-off in shares as an overreaction, though not necessarily surprising given sentiment recently. Given recent focus on/concern around competition for LentiGlobin [BB305], on top of the overall skittishness of the biotech market, we aren't surprised to see this knee-jerk reaction, though we think it's overdone and would expect shares to rebound as investors have time to actually digest the update."

Bluebird shares were down as much as 18% earlier Monday.

Bluebird will be presenting updated results from its ongoing gene therapy clinical trials at the American Society of Hematology annual meeting in December. 

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.