SAN FRANCISCO (TheStreet) -- Bluebird Bio (BLUE) has now treated seven beta-thalassemia patients with its experimental, one-time gene therapy. Four of the patients -- all followed for longer than three months -- are producing enough oxygen-carrying hemoglobin on their own to eliminate the need for chronic blood transfusions.

Two of these super-responding beta-thalassemia patients -- followed for a year and nine months, respectively -- have hemoglobin levels of healthy adults. At this point, a single infusion of Bluebird's gene therapy has essentially cured them of this serious, inherited blood disease.

The remaining three beta-thalassemia patients were infused with Bluebird's gene therapy around one month ago so it's too early to assess their response. A single patient with sickle cell disease was also just treated within the past month.

It's still unreasonable to expect an equivalent strong response in every patient, but Bluebird is learning that as its therapy replaces the defective gene causing beta-thalassemia (or sickle cell disease) with a gene that is fully functional, the patient's ability to produce normal-functioning hemoglobin improves over time, said CEO Nick Leschly.

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Bluebird Gene Therapy Inducing Durable Cures in Blood Disorder Patients