The researchers’ hope was that the modified stem cells would mature into red blood cells and produce robust amounts of healthy hemoglobin. That hope was realized. 9 of the 2 patients suffered from severe beta thalassemia, and, after treatment, the number of blood transfusions they required fell by 74%. 3 of the 9 no longer need any transfusions at all. The same is true of 12 of the 13 patients with the less severe version of the disease. So far, the subjects of the trial have been observed for a maximum of 42 months, but they will be monitored long into the future, to insure that the benefits of the therapy persist and cause no serious side effects. 1 early concern—that the procedure could disrupt the DNA of the stem cells, potentially triggering leukemia—has not, fortunately, come to fruition.

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